point of departure for much work and for a large series of objections
directed against the phagocytic theory. The Hungarian investigator found
that the defibrinated blood of the rabbit was capable of destroying _in
vitro_ a great number of anthrax bacilli. From this it was concluded
that the fluids of the living body possessed a bactericidal power
sufficient to explain the immunity against infective micro-organisms.
The destruction of the anthrax bacillus by defibrinated blood was
confirmed by a young American investigator of great talent,
Nuttall[880], who carried out an important work on this subject in the
laboratory and under the direction of Flügge at Breslau. He was able to
follow step by step, by the observation of anthrax bacilli on the warm
stage, their degeneration under the action of the defibrinated blood.
This destruction of the bacilli took place outside the phagocytes. The
same phenomenon could be shown by the method of gelatine plate cultures.
The bacilli, subjected to the influence of the defibrinated blood of
rabbits and other vertebrates, usually died or were markedly injured.
The blood when heated to 55° C. completely lost its bactericidal power.
[Sidenote: [549]]
These observations, perfectly exact in every detail, gave Flügge[881]
and his assistant Bitter[882] the opportunity to criticise vigorously
the theory of phagocytosis. The cells were said to be incapable of
ingesting living micro-organisms; these latter must be previously
destroyed by the bactericidal action of the body fluids, and it was only
their dead bodies which were devoured by the phagocytes.
Flügge based his criticism upon considerations of a general character
and upon observations made mainly by Nuttall. “There is no necessary
point of analogy,” says the learned Breslau hygienist, “between the
ingestion of food and the struggle against infective micro-organisms,
nor between nutritive substances and living micro-organisms” (p. 225).
“From Nuttall’s results it must evidently be accepted as possible that
the phagocytes can ingest dead bacteria only and that they have not the
power of ridding the body of the living infective agents” (p. 226). The
following passage is especially significant. “When we examine, with an
open mind, a series of preparations which show the relations between the
phagocytes and the bacteria in various infective diseases, the
phagocytes sometimes present themselves as the victims of the bacteria,
which continue their triumphal march; sometimes they produce the
impression of tombstones lying in large numbers behind the line of
battle and after the end of the struggle. On the other hand, they in no
way force themselves upon our notice as instruments of slaughter which
the attacked organism makes use of to defend itself” (p. 227).
These arguments have been regarded by many investigators in all
countries as perfectly sufficient to overthrow the phagocytic theory.
The bactericidal power of the body fluids became the rallying cry of a
great number of works always directed to the same object: to replace the
rôle of phagocytosis by that of a bactericidal power of the body fluids.
It is quite unnecessary to weary the reader with a list of the very
numerous publications that have appeared on this subject in every
European language. But it is not possible to pass over in silence the
work of some of the principal partisans of the humoral theory of
immunity.
[Sidenote: [550]]
The first place amongst these works certainly belongs to von Behring’s
memoir[883] on the natural immunity of white rats against anthrax. As
already stated in Chapter VI of this work, von Behring discovered the
very remarkable power possessed by the rat’s blood of destroying anthrax
bacilli with very great rapidity. This investigator did not hesitate to
conclude therefrom that this bactericidal property of the blood must, in
the rat, bring about a great resistance against anthrax. We should have
in this case, then, an example in which the immunity did not depend in
any way upon phagocytosis, but would be bound up entirely in a purely
humoral property.
With the object of deciding whether the bactericidal property of the
blood is really the general and essential cause of natural or acquired
immunity, von Behring, in collaboration with Nissen[884], carried out a
long series of experiments, the results of which, however, did not
confirm their expectations. They found that in animals well vaccinated
against certain bacteria (notably Gamaleia’s vibrio or _V.
metschnikovi_), the blood plasma undoubtedly acquires a high specific
bactericidal power, but at the same time they satisfied themselves that
the blood, even of well immunised animals, was generally incapable of
killing the micro-organisms. The bactericidal property, then, according
to their researches, presented itself not as a general character but as
one of limited importance. These facts even led von Behring to abandon
the theory of the bactericidal power of the body fluids as an
explanation of immunity.
This theory found many warm partisans, especially at Munich. Emmerich
had already announced at the International Congress of Hygiene, held at
Vienna in 1887, that in the blood of rabbits vaccinated against the
bacillus of swine erysipelas an antiseptic substance of remarkable
activity is produced. To this, exclusively, in this instance, and not to
the phagocytes, he attributed the acquired immunity. Later,
Emmerich[885] in an investigation carried out in collaboration with di
Mattei developed this view. We may refrain from giving any account of
the contents of their memoir as well as from criticising their
conclusions, as this has already been done in Chapter IX. Let us content
ourselves with stating that our own experiments, as well as those made
later by Mesnil, have demonstrated the inaccuracy of Emmerich’s
statements.
[Sidenote: [551]]
Another Munich bacteriologist, H. Buchner, at first expressed
himself[886] very favourably on the theory of phagocytosis. He regarded
it as more capable of explaining most of the phenomena of immunity than
was his own older local theory. But little by little he declared himself
in formal opposition to the cellular theory of immunity and went over to
the camp of his sometime adversaries. He adopted[887] the humoral theory
of the bactericidal action of the body fluids, upon which subject he
carried out several important investigations. He was able without
difficulty to confirm Nuttall’s discovery of the disappearance of the
microbicidal power when the defibrinated blood was heated to 55° C., and
he added to this fundamental fact many others of great value. He
demonstrated the part played by the salts in the exercise of this
bactericidal power, and laid great stress on the fact that this power
depends on the presence of a special substance of albuminoid nature, to
which he gave the name of _alexin_. Buchner[888] combatted with success
the idea that I had expressed, according to which the bactericidal power
of the body fluids is reduced in great part to a plasmolytic action of
the blood serum upon certain micro-organisms. It cannot be denied that
my hypothesis is only very partially applicable, and that the larger
share in the bactericidal action of the body fluids belongs to the
alexins. Buchner also made the study of this action more easy by the
demonstration that the red blood corpuscles of a foreign species
undergo, under the action of the blood and of the serums, a globulicidal
action comparable to that which occurs in the case of micro-organisms.
Whilst Flügge, von Behring and many others of the old partisans of the
bactericidal theory of the body fluids abandoned it more or less
completely as an explanation of immunity, Buchner remained faithful to
it and tried, aided by the collaboration of his pupils, as far as
possible to defend it.
[Sidenote: [552]]
In France this humoral theory was adopted chiefly by Bouchard[889] and
his pupils, amongst whom I must cite more particularly Charrin and
Roger. They sought to confirm it by personal researches, the greater
part of which were carried out upon the bacillus of blue pus. These
investigators studied it especially in relation to acquired immunity. A
comparison of the mode of development of the pyocyanic bacillus in the
serum of susceptible animals and of vaccinated animals of the same
species, convinced them of the great importance of the action of the
body fluids. In cases where these fluids were found to be incapable of
killing the micro-organisms they exerted over them an injurious
influence, either by attenuating their virulence, or by producing more
or less important modifications in their forms and functions. The
essential cause of natural or acquired immunity was always attributed by
Bouchard’s school to the property of the body fluids. The phagocytes
were said to intervene only secondarily, either to carry off the dead
bodies of the micro-organisms, or to ingest the bacteria, rendered
inoffensive by the humoral action.
The humoral theory of immunity, with some slight modifications, spread
very generally into every country, and many investigators accepted it
without reserve. But certain observers ventured to run counter to the
general current and raised objections of principle against the theory of
the bactericidal power of the fluids of the body. After the principal
facts established by the partisans of this theory had been confirmed, it
was asked whether the phenomena of the destruction of micro-organisms
observed _in vitro_ are really equivalent to those produced in the
refractory animal. A glance at the data brought together with so much
zeal was sufficient to demonstrate that this parallelism does not exist.
The blood of animals susceptible to certain micro-organisms was found to
be bactericidal for these organisms, whilst that of refractory animals
was incapable of destroying them. It is useless to cite examples, so
numerous are they. On the other hand, the bactericidal power of the body
fluids, so marked for certain pathogenic organisms such as the anthrax
bacillus and especially the cholera vibrio and the typhoid
coccobacillus, is insignificant or _nil_ as regards many bacteria
against which refractory animals are not wanting.
[Sidenote: [553]]
All these facts throw doubt on the predominating part played in immunity
by the bactericidal power of the body fluids. Lubarsch[890] attacked the
humoral theory, showing by a great number of experiments that animals
whose fluids are very bactericidal _in vitro_ are very susceptible to a
much smaller quantity of bacteria of the same species introduced into
the body. Thus, the defibrinated blood and the blood serum of rabbits
destroy a large number of bacteria in a very short time, whilst the
rabbits themselves contract fatal anthrax after the introduction of a
small number of these micro-organisms into the blood vessels. This
contradiction cannot be explained except by the profound changes which
the blood must undergo outside the body. Facts of the same nature have
been shown for the anthrax of rats by Hankin, Roux, and ourselves, as
described in Chapter VI.
[Sidenote: [554]]
The International Congress of Medicine, assembled at Berlin in 1890, was
the first occasion on which I spoke publicly of the new theories of
immunity. In the addresses given at the general meetings, leaders of
medical science in several countries summed up their opinion on this
question. Koch[891], in his memorable report, declared that the new
acquisitions had destroyed the basis of the theory of phagocytes, and
that consequently it must give place to the humoral theory of immunity.
Bouchard took up a more conciliatory position, but, according to him,
the bactericidal power of the fluids of the body was the primary and
essential cause of immunity. The phagocytes only intervened later, in
order to finish the work begun without their assistance. Lord Lister
expressed himself[892], on the other hand, much more favourably on the
subject of the theory of phagocytosis. This observer, who is not only a
great surgeon, but is perhaps even more remarkable for his great powers
of generalisation, has paid special attention to the problem of
immunity. With the object of clearing up this very complicated and at
the same time important question, Lord Lister seized the occasion of the
meeting of the International Congress of Hygiene in London in 1891, to
bring about an exchange of views between the partisans of the various
theories of immunity. Under his presidency he devoted an entire sitting
of the Section of Bacteriology to the discussion of this question.
Buchner presented a report[893] drawn up exclusively from the point of
view of the humoral theory and devoted to the demonstration of the
slight importance of phagocytosis, and also to the preponderant part
played by the alexins dissolved in the body fluids and circulating in
the plasma of the blood. He attempted to harmonise the facts on the
bactericidal power of serums observed _in vitro_ with the special
conditions to be met with in the animal body. He specially insisted on
the point that, in the blood and the organs, the alexins cannot act with
the same rapidity that they can in test tubes containing serum. In this
way he recognised that between the bactericidal action _in vitro_ and
that in the body of the animal, there exists a marked difference, but he
would not consent to attribute it in the latter case to the intervention
of the phagocytes.
Roux[894] also made a report on immunity at the same sederunt, speaking
very distinctly in favour of the cellular theory. A chemist by
inclination, he was sympathetic at first to the humoral theories of
immunity. Working with Pasteur, and side by side with him, Roux, from
the beginning of the new era of medical science, had made numerous
experiments on the part played by the body fluids in immunity. But as
the results were not sufficiently precise and demonstrative they were
soon abandoned. The attachment of Roux, however, to the humoral theories
was manifested in his work, carried out in part with Chamberland[895],
on the subject of vaccination by means of microbial products. Later,
having obtained a deeper knowledge of various facts concerning natural
and acquired immunity, he rallied to the cellular conception and
developed it in his report presented to the above Congress in London.
Several microbiologists took part in the discussion, and I myself[896]
was able to communicate certain facts concerning the immunity of
guinea-pigs, acquired as the result of vaccination against Gamaleia’s
vibrio. I chose this example because it presented, according to von
Behring and Nissen, the clearest case of a bactericidal property
developed during the course of immunisation. I was able to furnish the
proof that, in the vaccinated animal, the micro-organism in question, in
spite of the great bactericidal power of the blood serum _in vitro_,
remains alive in the animal body for a long time, and that its
destruction is effected by the phagocytes, which ingested it alive. In
this example I showed that the leucocytes of the exudation, that have
ingested vibrios, may still furnish cultures of this organism if they
are taken from the body and transferred in hanging drop to the
incubator.
[Sidenote: [555]]
The fact that, even in the case which appeared most to favour the
humoral conception of acquired immunity, phagocytes play the principal
part, must to many members of the Congress have appeared sufficiently
significant. Indeed, several observers who were present at the debates,
received the impression that the phagocytic theory had not been
overturned by its adversaries. At this period the question of the
importance of antitoxins from the point of view of immunity had scarcely
been raised. The great discovery made by von Behring and Kitasato was
already accepted by everyone; but there was no ground for attributing to
it any general importance. In fact, though proved for tetanus and
diphtheria, and extended by Ehrlich’s beautiful experiments to the
vegetable toxins (ricin, abrin, and robin), the antitoxic property of
the fluids of the body presented itself rather as a special than as a
general phenomenon. It is in this sense that Roux had assigned to it its
place in the chapter of immunity. The two diseases, against which
antitoxic serums had been discovered, are certainly distinguished from
the great majority of infections by the localisation of the
micro-organisms and the abundant secretion of their toxins.
It was only after the London Congress that this question came
prominently forward. Von Behring thought that the antitoxic power of the
body fluids is generally distributed in all cases of acquired immunity,
and that micro-organisms, introduced into the animal possessing this
power, become incapable of any pathogenic manifestation. Certain facts,
brought together in Bouchard’s laboratory, tell against the hypothesis I
have just mentioned. With the object of throwing light on this question
I began, immediately after the close of the Congress, to study the
acquired immunity of rabbits against the micro-organism of the
pneumo-enteritis of pigs. I was able to demonstrate[897] that in this
case the resistance of the animal against the micro-organisms does not
depend on the acquisition of any antitoxic property by the body fluids;
such a property is completely absent. At the same time I showed that the
serum of vaccinated rabbits possesses a very marked protective power
against infection by the coccobacillus of pneumo-enteritis. It was for
the first time proved that independently of the antitoxic and
bactericidal properties of serums, there exists another special
property, the anti-infective property. This I conceived to be of the
nature of a stimulant action on the part of the phagocytes.
[Sidenote: [556]]
It has already been stated in an earlier chapter that before the
discovery of antitoxins Richet and Héricourt[898] had observed an
immunising action of the serum of animals refractory to staphylococci.
These observers were content with this demonstration and did not seek to
penetrate more deeply into the mechanism of the action of their serum.
For this reason when von Behring and Kitasato announced their discovery
of antitoxic serums it was generally thought that the antistaphylococci
serums were also antitoxic serums. The immunity against the
micro-organism of the pneumo-enteritis of pigs taught us that here we
might have to deal with quite a different matter. It was soon
demonstrated that the serum from the immunised animal might in fact,
without being antitoxic, present the same anti-infective property as in
the case of pneumo-enteritis. That was first proved in the case of the
experimental disease set up by Koch’s cholera vibrio.
[Sidenote: [557]]
The reappearance of cholera in Europe in 1892 drew the attention of
bacteriologists to this disease, and was the occasion of many new
researches on immunity against the cholera vibrio. Several important
works on this question were published by Pfeiffer[899], at this period
director of the scientific staff of the Koch Institute at Berlin. He
obtained, in animals well immunised against the cholera vibrio, a serum
endowed with a high anti-infective power but entirely without any
antitoxic property. The guinea-pigs themselves, very resistant to the
cholera peritonitis, were found, on the other hand, to be very
susceptible to the minimum lethal dose of the cholera poison. The
absence of antitoxic power in the fluids of the body taken in connection
with a well-marked phagocytic reaction in a large number of cases of
immunity, natural and acquired, has turned the scale in favour of the
cellular theory. The impossibility on the part of those who maintain the
purely bactericidal theory of the body fluids, to reply to the
objections above mentioned has accentuated this favourable movement.
Just at this moment, when the theory of phagocytes might be regarded to
have obtained the rights of citizenship, a discovery was made which
appeared to overturn it completely. I have mentioned more than once that
the attempts of the partisans of the bactericidal theory of the body
fluids have failed whenever it was necessary to give evidence of their
action in the refractory animal. Instead of a destruction of the
micro-organisms in these fluids, it was always found that they perished
inside the phagocytes. These facts have even led to the manifestation of
a desire to harmonise the humoral theory with the theory of
phagocytosis. Denys, with certain of his collaborators, and Buchner and
his pupils came to the conclusion that the alexins are merely leucocytic
products. As regards the theory of phagocytosis we have this section,
who attribute an important function in healing and immunity to the
emigration of the leucocytes towards, and their accumulation at the
menaced spot. They admit that the leucocytes really represent the
healing elements of the animal body; it is not, however, they say, their
phagocytic functions which confer upon them this rôle but their power of
secreting alexin. These bactericidal substances act outside the
phagocytes—in the plasmas of the blood and of the exudations—and
phagocytosis only intervenes at a later period and secondarily.
This new modification of the bactericidal theory of the body fluids has
often been termed by Buchner a connecting bridge between the humoral
theory and the cellular theory of immunity.
In the midst of this movement of conciliation, Pfeiffer[900] in 1894
published a work on the immunity of the guinea-pig against experimental
cholera peritonitis. He maintains that here the destruction of the
vibrios takes place without any co-operation on the part of the
phagocytes and exclusively by means of the body fluids. The vibrios,
before their complete destruction and solution in the fluids of the
body, are transformed into granules, presenting the transformation to
which we have given the name of Pfeiffer’s phenomenon.
[Sidenote: [558]]
Several of Pfeiffer’s pupils have confirmed his view in connection with
the cholera vibrio, and have extended it to several other
micro-organisms such as the typhoid coccobacillus. The destruction of
the micro-organisms in these cases is brought about, according to
Pfeiffer and his collaborators, not by the alexins of Buchner, but by a
separate substance. The protective anti-infective serum contains it in
an inactive state only; but immediately this serum is introduced into
the body of a normal animal, the bactericidal substance is acted upon by
the endothelial cells and becomes “active,” capable of destroying a
large number of vibrios. Pfeiffer has developed this theory more
especially in an article published in 1896, entitled “Ein neues
Grundgesetz der Immunität[901].” Pfeiffer’s observation and his theory
built upon it gave a new lease of life to the humoral theory and for
some time many observers believed that the theory of phagocytosis was
now finally overturned. Fränkel[902] announced, in a public address,
that science in its progressive march has “discovered the methods of
defence employed by the animal organism against its most dreaded
enemies, methods which have nothing in common with phagocytosis, which
act quite independently of the phagocytes and manifest an action so
energetic that we may calmly eliminate all other factors.” This view is
based on the discovery of antitoxins and the bactericidal substance
studied by Pfeiffer.
It will be readily understood that as soon as I learnt of the existence
of a real extracellular destruction of micro-organisms I at once began
to study it in order to find out its real importance amongst the
phenomena of immunity. First of all, I examined Pfeiffer’s phenomenon in
connection with the cholera vibrio[903], and I was able to show that it
was produced only under special conditions. The pre-existent phagocytes
must be greatly injured before the cholera vibrios can be transformed
into granules. Phagolysis (so I termed this transitory damage to the
phagocytes) is indispensable for the manifestation of Pfeiffer’s
phenomenon in the peritoneal fluid. When it is suppressed, by preparing
the phagocytes by means of injections of various fluids, we find that,
instead of Pfeiffer’s phenomenon, phagocytosis is almost instantaneously
produced. In positions where very few or no leucocytes are pre-existent,
as in the subcutaneous tissue, Pfeiffer’s phenomenon is never observed.
[Sidenote: [559]]
Even in the case of the cholera vibrio the extracellular destruction is
observed, therefore, only in special cases. Most of the other pathogenic
micro-organisms do not undergo this destructive process at all under
conditions in which the cholera vibrio exhibits Pfeiffer’s phenomenon in
a marked degree. These facts appeared to justify me in the conclusion
that the destruction of micro-organisms takes place in the animal body
by means of soluble ferments, the result of phagocytic digestion. These
ferments are found under the normal condition within these phagocytes
and escape from them when they are destroyed or receive some transient
injury. This conclusion was in flat contradiction to the theory and
statements of Pfeiffer, who attributed an important function to the
endothelial secretions. To settle this controversy I tried to obtain
Pfeiffer’s phenomenon outside the body, that is to say independently of
any co-operation from the peritoneal endothelium. It is sufficient to
add a little peritoneal lymph, rich in leucocytes, to the inactive
anti-infective serum, to obtain in hanging drops the transformation of
the cholera vibrios into granules.
Bordet[904], in my laboratory, repeated this experiment with the object
of determining its essential mechanism. He succeeded in obtaining
Pfeiffer’s phenomenon _in vitro_, not only by adding peritoneal lymph
from a normal guinea-pig to the specific serum, but also by adding to it
a drop of fresh blood serum from the same animal. The analysis of the
phenomena which take place under these conditions led Bordet to the
following hypothesis. The destruction of micro-organisms in vaccinated
animals takes place by the co-operation of two substances. One of these
is Buchner’s alexin which is found normally in the phagocytes; it sets
up bacteriolysis properly so-called when it is enclosed within the
leucocytes or after it has escaped from them at the time of phagolysis.
To attain this end, however, the alexin needs the co-operation of
another substance. This is the protective or sensibilising substance of
Bordet. It circulates in the plasmas and carries a specific character
which is absent from the alexin. I need not here insist at any length on
this theory, because it has already been sufficiently explained during
the course of this work.
[Sidenote: [560]]
The data on the restricted part played by Pfeiffer’s phenomenon and on
its mechanism, above summarised, have been attacked by Pfeiffer and by
several other observers, but they have received general confirmation, so
that their accuracy can no longer be in doubt. Objections were also
raised to Bordet’s view of the mechanism of bacteriolysis. Thus, Abel
has criticised it in the following argument[905]: “In spite of the
soundness and the boldness of the majority of Bordet’s statements on the
importance of the various factors, and especially of the leucocytes in
immunity, it cannot be doubted that later researches will modify and
correct his interpretations which we, in Germany, do not accept in their
full extension. Up to the present, the victory in the various rounds has
always been with Pfeiffer, whose researches, solid and exempt from bias,
have made him, to use a sporting expression, the ‘favourite’ with all
those who follow attentively the international contest in the arena of
the problem of immunity.” Abel is certainly a highly esteemed
bacteriologist, but he is not a good prophet, and he assumes a mistaken
attitude in looking at the subject from a “national” point of view[906].
In Germany much interest is taken in scientific movements and, very
naturally, original and new theories are there criticised and discussed.
But that does not justify one in putting forward against an opinion the
statement that it is not accepted in Germany. In this country, so rich
in scientific work, we find partisans of the most opposite views. In any
case, in the conflict between Pfeiffer on the one hand, and Bordet and
myself on the other, things have not turned out as Abel predicted. The
two substances which act in the destruction of the micro-organisms are
now accepted by the whole world. The intimate relations between the
alexins and the leucocytes are equally recognised by very many
observers. The fact that the alexins are confined within phagocytes has
been confirmed by several observers, and has received a very convincing
proof from Gengou’s experiments on the comparative action of the serum
and blood plasma against micro-organisms. The existence of phagolysis,
denied at first by some observers, has been verified by others and can
now no longer be doubted.
[Sidenote: [561]]
The relations between the sensibilising substance and the phagocytes are
less easily grasped than are those between the alexins and the
leucocytes. Nevertheless, the experiments made by Pfeiffer and
Marx[907], have led these observers to recognise that the former arise
from the spleen, the lymphatic glands, and the bone marrow, that is to
say, organs which are pre-eminently phagocytic. This result has been
confirmed by Deutsch and must be regarded as definitely settled. All the
data collected in recent years have, therefore, confirmed the view that
the destruction of micro-organisms in the refractory animal presents
itself as a special example of their absorption by formed elements. This
truth was so fully recognised in our laboratory that the analogy between
bacteriolysis and the destruction of animal cells was looked upon as
quite natural and evident. Bordet had for some years past observed that
the blood serum of certain animals presented a marked analogy in its
agglutinative property in regard to micro-organisms and in that against
red blood corpuscles. In 1898, studying the fate of the spirilla of the
goose in the peritoneal cavity of guinea-pigs (see Chapter VI), I
observed that these micro-organisms underwent the same changes both
within and outside the phagocyte; this fact appeared to me to be in
perfect harmony with the whole of our knowledge concerning the
absorption of formed elements and on intracellular digestion.
Bordet[908], prepared by his preceding researches on the agglutination
of the red blood corpuscles, set himself to study the fate of the red
corpuscles in the animal body. He easily established a close
relationship between the development of the bacteriolytic property and
the haemolytic power of the serum of animals prepared by repeated
injections of bacteria and of blood. His results were soon (January,
1899) confirmed by Ehrlich and Morgenroth[909], who supplemented them
with the important statement that Bordet’s sensibilising substance, or
intermediary substance (E. and M.), has the property of attaching or
fixing itself to the red blood corpuscles.
[Sidenote: [562]]
The works on haemolysis, carried out during the last three years by
Ehrlich and Morgenroth on the one hand, and by Bordet on the other, have
allowed us to extend our study of the mechanism of the action of the two
substances on micro-organisms and on animal cells. Ehrlich has extended
his ingenious theory of antitoxins to the bacteriolytic substances,
which he regards as side-chains detached from the cells and capable of
absorbing the toxins. In a series of remarkable investigations, most of
them carried out in collaboration with Morgenroth, Ehrlich has developed
his theory which attempts to offer an account of the essential mechanism
which presides over the destruction of micro-organisms and over the
neutralisation of their poisons. This theory is at present in full swing
of development. Some of his points contradict several of the conclusions
in Bordet’s works. Whilst the latter maintains that the sensibilising
substance becomes fixed as a mordant, Ehrlich regards it as entering
into chemical combination with the molecular group of the
micro-organisms and of the animal cells. According to Bordet, the alexin
of the same species of animal is always the same substance. Ehrlich
energetically maintains the plurality of the alexins, to which he gives
the name of complements.
This controversy has caused a most interesting exchange of views and has
led to experiments which are remarkably ingenious, but it must be
admitted that as yet all the points in dispute are not definitely
settled. It is evident that we have here a new line of research which
promises most fruitful results for science.
We have described in various chapters of this work the fundamental
elements of Ehrlich’s theory. Many think that this theory is, in
principle, antagonistic to the theory of phagocytosis, but we have
already observed that this view cannot be accepted. It is true that
Ehrlich maintains that the bacteriolytic and cytotoxic ferments which we
have called _cytases_ (alexins or complements) circulate in a state of
solution in the blood plasma, whilst, according to the theory of
phagocytosis, they are found under normal conditions inside phagocytes.
But this view has nothing to do with the basis of the theory of
receptors, or of Ehrlich’s side-chain theory, according to which the
antitoxin and certain other antibodies (intermediary substance) are
regarded as products detached from cells having an affinity for the
toxins and the microbial products.
The theory of phagocytosis seeks to establish the part played by these
cells in the destruction of micro-organisms. It maintains that the vital
manifestation of the phagocytes, irritability, mobility, and voracity,
constitutes an essential factor in ridding the animal of
micro-organisms, because the true bactericidal ferment is contained
within the phagocytes, except in cases of phagolysis. The destruction of
the micro-organisms follows the laws which govern the absorption of
formed elements in general. This absorption, finally, is the work of two
soluble digestive ferments, one of which (fixative) is readily excreted
by the phagocyte into the plasmas of the blood and exudations. The
theory of phagocytosis seeks to establish these principles with the
greatest possible exactness, but it has not yet ventured to penetrate
more deeply into the phenomena of intracellular digestion which are
confounded with the action of soluble ferments in general. This problem
is still far from being satisfactorily solved.
[Sidenote: [563]]
In spite of very numerous objections, of which the principal ones have
already been mentioned, the theory of phagocytosis, within the limits
indicated, so far from being overturned, has become more and more
consolidated, thanks to the numerous observations made since its
foundation. It is for this reason that the opposition has calmed down of
late years and that in many works the opinions expressed have become
more favourable to the rôle of phagocytosis in immunity.
Soon after the Congress of Hygiene in 1891, the Pathological Society of
London devoted several meetings to a discussion of the question of
immunity. Many eminent observers took part in these debates, which were,
in general, favourable to this theory of phagocytosis[910].
At the International Congress of Hygiene, held at Budapest in 1894, the
question of immunity was again discussed. Buchner[911] made a report in
which he specially insisted on the leucocytic origin of the alexins,
regarding this fact as particularly capable of reconciling the
bactericidal property of the body fluids with the theory of
phagocytosis. The alexins, however, secreted by the leucocytes, must, it
was assumed, carry out their principal function in the plasmas of the
blood and exudations. Phagocytosis would only intervene secondarily for
the purpose of ingesting the micro-organisms which had been already
killed or seriously injured by the alexins of the body fluids.
[Sidenote: [564]]
In his last summary of the question, presented to the International
Congress of Medicine at Paris in 1900, Buchner[912] maintains his theory
of leucocytic secretions. But he already takes one step more towards the
theory of phagocytosis, at least as regards natural immunity. He
consents to accept the fact “that phagocytic activity is in many cases
of decisive importance in overcoming the infective processes, especially
in those cases in which the secreted alexins were unable to bring about
more than a temporary attenuation of the vital functions of the
bacteria. Under these conditions the bacteria could only be modified in
so far as their chemical functions were transformed into a latent state,
from which they would be ready to regain their full vital activity
should it happen that the phagocytes were not there to prevent them from
doing so.” In any case this view is widely removed from the old theory,
according to which phagocytes were regarded as capable of ingesting dead
and inoffensive bacteria only.
A second adversary of the theory of phagocytosis, von Behring[913],
gives a place to this theory not only in certain examples of natural
immunity but even in some cases of acquired immunity, e.g. in the
immunity of sheep vaccinated against anthrax, an example I have already
cited in Chapter VIII (cf. _supra_, p. 242).
It would take too long to describe the change of opinion on the theories
of immunity that has taken place during recent years. I will content
myself with citing certain examples which shall be taken from the works
of declared adversaries of the theory of phagocytosis. Thus, Flügge, who
early declared against the cellular theory completely and categorically
and at the same time argued strongly in favour of the humoral theory,
has been gradually led to depart from his first position. We may follow
the steps of his conversion in the different editions of his _Outlines
of Hygiene_. In the first edition published in 1889 he expresses himself
in the following manner[914]: “Recent researches indicate the
probability, however, that the phagocytes in by far the greater majority
of cases seize the infective agents which, already dead, are not in a
condition suitable for the performance of a defensive function. On the
other hand, it is proved that the blood and blood plasma of warm-blooded
animals possess the property of destroying, very quickly, enormous
numbers of pathogenic bacteria,” ... etc. In the fourth edition of the
same work, published in 1897, we find at the corresponding place the
following passage[915]: “Recent researches indicate the probability,
however, that the theory of Metschnikoff ... is not in a position to
offer a complete explanation of the process of immunity.” This passage
is followed by a somewhat conciliatory and eclectic development of the
theory.
[Sidenote: [565]]
Let us take as a second example Günther’s _Introduction to the Study of
Bacteriology_, widely read both in the original and in translations. In
the first edition published in 1890[916] the theory of phagocytosis is
curtly dismissed as “being incapable of withstanding criticism.” In the
fifth edition of the same work, however[917], published in 1898, this
theory is no longer treated thus summarily. It is given a place amongst
the theories of immunity and an attempt, similar to that made by
Buchner, is made to reconcile it with the humoral theory.
A change in the same direction may also be observed in Charrin’s view.
In the first edition of his _Pathologie générale infectieuse_, this
observer[918] had already taken an eclectic view on this question of the
theories of immunity. But the function which he assigns to the
phagocytes is subsidiary and secondary, whilst to that of the humoral
properties is assigned a position of primary importance. In the second
edition of the same work, which appeared seven years later[919], the
importance of phagocytosis is recognised in a much larger measure, as
may be gathered from the following passages: “For my part, I have always
accepted phagocytosis: at the same time I have always accepted the
existence of special humoral properties. As early as 1888 I showed, _in
vivo_, that the germs are modified outside the cells; but I did not know
from what groups of anatomical elements these properties were derived, I
exaggerated their importance and it is the decision of this origin and
this importance that renders it possible to reconcile the two theories”
(p. 250). “After all, the defence rests upon these two great processes
or cellular activities, phagocytosis in the first line, and then humoral
influences, some of them bactericidal and injurious to the living germ,
others antitoxic and injurious to their secretions” (p. 253).
[Sidenote: [566]]
Whilst the theory of phagocytosis has been consolidated by the
demonstration: (1) that the phagocytes, in cases of immunity, ingest and
destroy the living and virulent micro-organisms without the latter
needing to be previously deprived of their toxins; (2) that the
phagocytes absorb toxic substances; (3) that the phagocytes contain
bactericidal cytases and produce fixatives; the humoral theories, in
spite of all the efforts made to defend them, could never be developed
as theories that were in the slightest degree of general application.
Certain observers who from the first were very sympathetic to the
humoral theories have attempted to give a complete summary of these
properties. Thus, Stern[920] and later Frank[921] have published reports
drawn up with great care and in a very impartial spirit on the works
treating of the properties of the body fluids and the part they play in
immunity. This is how they sum up the question. Stern came to the
conclusion that it is impossible “to demonstrate at all regularly the
existence of relations between the bactericidal action of the blood and
immunity in all the infective diseases. In some cases, however, these
relations are so marked that, for these examples, a causal bond between
the two factors is extremely probable.” Frank expresses himself in the
following manner: “It follows most clearly that the immunity of an
animal—immunity innate or acquired—corresponds with the bactericidal
property of the blood in certain exceptional cases only. The only
animal, absolutely susceptible to anthrax and whose blood is entirely
without any bactericidal power, that it is at present possible to cite,
is the mouse.” “The bactericidal action of the blood serum is
undoubtedly a fact of great biological importance; but equally certainly
it cannot be the general cause of immunity, whether innate or acquired.”
An attempt was made to give fresh life to the humoral theory, either by
assuming that the bactericidal substance is nothing but the eosinophile
or pseudo-eosinophile secretion of the leucocytes (Kanthack), or by
supposing that for the destruction of micro-organisms in the animal body
the intervention of the agglutinative substance dissolved and
distributed in the body fluids is essential (Max Gruber). These two
views were put forward in a tentative form and as preliminary
communications only; there is no possibility of raising them to the
dignity of theories, and of late years they have not been upheld.
It cannot be denied that not one of the humoral theories has been able
to retain its position or to stand against the numerous facts that have
been accumulated during recent years.
This extraordinary discrepancy between the bactericidal power of the
body fluids and immunity is explained by the circumstance that the
microbicidal substances exist in the living animal within phagocytes and
only escape from them when these cells have been injured. The fact, so
well demonstrated by Gengou, that the blood plasma is without any
bactericidal power has given the final blow to the microbicidal theory
of the body fluids and it can no longer be maintained.
[Sidenote: [567]]
The humoral theories, based on the antitoxic and protective power of the
body fluids, can claim only a very restricted application. These
properties are met with in acquired immunity only, and even there are
not constant. Many cases of acquired immunity against micro-organisms
are unaccompanied by any antitoxic power, and in several examples of
this immunity the body fluids do not exhibit any protective power.
There is only one constant element in immunity, whether innate or
acquired, and that is phagocytosis. The extension and importance of this
factor can no longer be denied.
It is clearly proved that phagocytes are susceptible cells which react
against morbific agents, whether organised or not. These cells ingest
micro-organisms and absorb soluble substances. They seize microbes
whilst these are still living and capable of exercising their noxious
effect and bring them under the action of their cellular contents, which
are capable of killing and digesting the micro-organisms or of
inhibiting their pathogenic action. Phagocytes act because they possess
vital properties and a faculty of exerting a fermentative action on
morbific agents. The mechanism of this action is not yet definitely
settled, and we can foresee that for future researches there will be a
vast and fertile field to be reached by pursuing this path.
The present phase of the question of immunity constitutes one stage only
in the development of biological science and one which is capable of
many improvements.
CHAPTER XVII
SUMMARY
Means of defence of the animal against infective agents.—Absorption of
micro-organisms.—Phagocytes, and their function in
inflammation.—The action of phagocytes in the absorption of
micro-organisms.—The cytases, phagocytic ferments.—The cytases are
closely bound up with the phagocytes.—The fixatives and their
function in acquired immunity.—The fixatives are excreted by the
phagocytes and pass readily into the fluids of the body.—Essential
mechanism of the action of the fixatives.—Adaptation of phagocytes
to destroy micro-organisms in acquired immunity.—Difference
between the fixatives and the agglutinins.—Antitoxins and their
analogy with the fixatives.—Hypotheses as to the origin of
antitoxins.—Cellular immunity is a fact of general
import.—Susceptibility and its rôle in immunity.—Applications of
the theory of immunity to medical practice.
[Sidenote: [568]]
When an animal remains unharmed in spite of the penetration of infective
agents it is said to be immune to the diseases usually set up by these
agents. This idea embraces a very great number of phenomena which cannot
always be sharply separated from allied phenomena. On the one hand,
immunity is closely connected with the process of cure, on the other, it
is related to the disease. An animal may be regarded as unharmed if the
penetration of a very dangerous virus sets up merely an insignificant
discomfort. Nevertheless, this discomfort is accompanied by morbid
symptoms, though they may be very slight. It is useless and impossible
to set up any precise limits between immunity and allied states.
Immunity presents great variability. Sometimes it is very stable and
durable; in other cases it is very feeble and transient. Immunity may be
individual or it may be generic. It may be the privilege of a race, of a
species.
Immunity is often innate, as is the case of the immunity which is called
natural. But it may also be acquired. This last category of immunity may
be developed either by natural means, after an attack of an infective
disease, or as a result of human intervention. The principal means of
obtaining artificial acquired immunity consists in the inoculation of
viruses and of vaccines.
[Sidenote: [569]]
Immunity is a phenomenon which has existed on this globe from time
immemorial. Immunity must be of as ancient date as is disease. The most
simple and the most primitive organisms have constantly to struggle for
their existence; they give chase to living organisms in order to obtain
food, and they defend themselves against other organisms in order that
they may not become their prey. When the aggressor in this struggle is
much smaller than its adversary the result is that the former introduces
itself into the body of the latter and destroys it by means of
infection. In this case it takes up its abode in its adversary in order
to absorb the contents of its host and to produce within it one or more
generations. The natural history of unicellular organisms, both
vegetable and animal, often presents to us these examples of primitive
infection.
But infection also has its counter. The attacked organism defends itself
against the little aggressor. It protects itself by interposing a
resistant membrane, or it uses all the means at its disposal to destroy
the invader. As a very large number of organisms, in order to obtain
nourishment, are obliged to submit their food to digestion by various
chemical substances, they utilise these substances in the struggle
against the infective agents. They digest them whenever they are able to
do so.
One of the most primitive of organisms, the plasmodium of the
Myxomycetes, which is composed of formless protoplasmic masses
intermediate between lower animals and plants, ingests foreign bodies of
various kinds. It often happens that it incorporates numerous bacteria
which are growing alongside it on rotten wood or elsewhere. The
plasmodium allows them to live for some time within its digestive
vacuoles. But in the end it digests them by means of its soluble
ferments, substances intermediate between pepsin and trypsin. Owing to
this digestive power the plasmodia are not attacked by bacterial
infections.
[Sidenote: [570]]
This example, taken from amongst the most simple organisms, may serve as
a prototype for the phenomena of immunity in general. At the
commencement of the study of this remarkable property of so many living
organisms it was thought that the pathogenic micro-organisms
encountered, within the refractory organism, a medium which did not
allow them to live, either because of the absence of certain nutritive
substances indispensable for their existence or because it contained
some substance injurious to micro-organisms. Very numerous and detailed
researches have demonstrated the incorrectness of these hypotheses.
There are, of course, certain pathogenic micro-organisms which are very
exacting as regards the medium in which they will grow. Some will
develop only in the presence of particular substances, whilst others are
extremely sensitive to the slightest traces of poisons. These, however,
are quite the exception. The great majority of pathogenic
micro-organisms belonging to the group of bacteria readily adapt
themselves to all kinds of culture media, and most of them live and
develop freely in the blood or other fluids of refractory organisms.
This, therefore, is not the cause of the immunity in such organisms. The
cause must be sought for amongst factors more closely connected with
life.
Wishing to penetrate more deeply into these phenomena the hypothesis was
put forward that the unharmed organism got rid of the infective
micro-organisms by expelling them to the outside along with the excreta.
It was maintained for a considerable time that the animal organism
possessed the means of causing pathogenic bacteria to pass into the
kidneys, whence they were eliminated by the urine. It had to be
acknowledged, however, that this elimination never takes place in cases
of immunity, and only comes into operation when the animal is ill and
the integrity of the renal filter is impaired.
The infective micro-organisms, after they have entered into the unharmed
organism, remain there for a longer or shorter period, and perish
without being expelled. This disappearance of the micro-organisms takes
place by the same mechanism that rids the plasmodium of those bacteria
which it has managed to ingest during its slow peregrinations over dead
leaves or rotten wood. The micro-organisms are absorbed into the
refractory organisms as the result of a true act of digestion. It is
very remarkable that the gastro-intestinal ingestion, so well provided
with means of rendering the most varied aliments soluble, is generally
incapable of digesting pathogenic or other micro-organisms. It is very
rare to meet with soluble ferments of the intestinal canal which are
capable of digesting microscopic organisms, especially bacteria.
Consequently this organ, so rich in digestive diastases, is generally
inhabited by a large number of bacteria and other micro-organisms.
[Sidenote: [571]]
Even in animals whose food contains large numbers of microorganisms,
_e.g._ the larvae of flies, the digestive juices are powerless to
destroy them. Nevertheless, there are organisms which feed exclusively,
or almost exclusively, on bacteria and which are quite capable of
digesting them. These are the Protozoa, such as the _Amoebae_ and
certain Infusoria, which, without any trace of a digestive tube, easily
bring about this result. _Amoebae_ can be grown on the surface of agar
by taking care to sow along with them bacteria for their nourishment. It
is only necessary to give them a single species of micro-organism, and
this may be selected from the pathogenic forms, such as the cholera
vibrio or the _Bacillus coli_. The _Amoebae_ ingest a number of these
bacteria in the living state. They then kill them and digest them in
their digestive vacuoles which contain, along with a little acid, a
ferment belonging to the trypsin group, the amoebodiastase.
The bodies of lower and higher animals, alike, are very rich in elements
which closely resemble the _Amoebae_. Sometimes these are to be found in
the epithelial cells of the digestive canal which put out protoplasmic
processes for the purpose of seizing food and transferring it to their
interior, where it is submitted to the action of digestive ferments.
Sometimes they are the cells disposed between the body wall and that of
the intestinal canal, which float freely in the fluids of the body or
are more or less fixed in the interstitial tissue. The animal kingdom
presents a great variety of these amoeboid elements, known under the
general name of phagocytes (cells capable of devouring solid bodies).
One of the most primitive arrangements of phagocytes is met with in
_Ascaris_ and its allies belonging to the group of the Nematoda. All the
organisation that these round worms possess consists merely of four, or
a few more, enormous cells attached to the body wall. These are
phagocytes which push out processes of enormous length, capable of
exploring the whole of the internal cavity of the body.
[Sidenote: [572]]
The majority of phagocytes circulate in the lymph and blood and pass
into the exudations. These white corpuscles have a comparatively uniform
structure in the Invertebrata and present themselves as small cells with
a nucleus and a protoplasm capable of amoeboid movements. In the
Vertebrata we meet with two great categories of white corpuscles, of
which one group resembles those of the Invertebrata in that they also
possess a single large nucleus and an amoeboid protoplasm. These are the
macrophages of the blood and of the lymph, and are intimately connected
with the macrophages of such organs as the spleen, lymphatic glands, and
bone marrow. Another group of white corpuscles in the Vertebrata is made
up of small amoeboid cells which are distinguished by having a nucleus
which, although single, is divided into several lobes. These are the
microphages whose chief peculiarity, the multi-lobed form of the
nucleus, must be regarded as an adaptation for the purpose of passing as
rapidly as possible through the walls of capillaries and small veins.
The diapedesis of the white corpuscles, their migration through the
vessel wall into the cavities and tissues, is one of the principal means
of defence possessed by an animal. As soon as the infective agents have
penetrated into the body, a whole army of white corpuscles proceed
towards the menaced spot, there entering into a struggle with the
micro-organisms. Aided by the special form of their nucleus the
microphages are the first to pass through the walls of the vessels. Each
of the several small lobes, into which the nucleus and its protoplasm is
divided, passes readily through the minute orifices between the
endothelial cells of the vessels. The macrophages follow the microphages
and become mixed in greater or less numbers with the exudations. But it
is not micro-organisms only which set up this inflammatory reaction
accompanied by the emigration and the accumulation of leucocytes. The
introduction of inert bodies and of aseptic fluids brings about the same
result. The phagocytes are, as a matter of fact, endowed with a special
susceptibility, which enables them to perceive exceedingly small changes
in the chemical or physical composition of the medium that surrounds
them.
The leucocytes, having arrived at the spot where the intruders are
found, seize them after the manner of the _Amoebae_ and within their
bodies subject them to intracellular digestion. This digestion takes
place in the vacuoles in which usually is a weakly acid fluid which
contains digestive ferments; of these a very considerable number are now
recognised.
Just as the _Amoebae_ and the Infusoria make a choice from amongst the
small organisms that surround them, so the leucocytes choose bodies
which are best suited to their use. The macrophages seize by preference
animal cells such as the blood corpuscles, the spermatozoa, and other
elements which are derived from animals. Among the infective
micro-organisms the macrophages have a predilection for those that set
up chronic diseases such as leprosy, tuberculosis, and actinomycosis and
also for those which are of animal nature. Into this last category come
the amoeboid parasites of malaria, Texas fever and the _Trypanosomata_.
The macrophages can also ingest the bacteria of acute diseases, but,
save in exceptional cases, their intervention is of little moment.
[Sidenote: [573]]
The microphages, on the other hand, appear to play their part specially
in acute infections. Their intervention against animal cells is _nil_,
or almost so. Thus they rarely seize the red corpuscles of the same or
of a foreign species of animal. They also appear to be repelled by
parasites of animal origin and by certain bacteria which set up chronic
diseases. Whilst the macrophages seize the bacilli of leprosy with great
avidity, the microphages ingest them only exceptionally.
The morphological and physiological differences between the two great
categories of mobile phagocytes (leucocytes), correspond to differences
in the composition of their soluble ferments. Just as the _Amoebae_
digest their prey by means of their amoebodiastase, a soluble ferment of
the group of trypsins, so the white corpuscles submit the foreign bodies
ingested by them to the action of what are now known as cytases. These
cytases (alexins or complements of other writers) are soluble ferments
which also belong to the trypsin group. They act in a medium which is
feebly acid, neutral, or feebly alkaline, and, like the amoebodiastase,
they are distinguished by a great sensitiveness to heat. When the
cytases are contained in fluids, a temperature of 55°–56° C. destroys
them rapidly and completely. When they are found in organs reduced to
the state of an emulsion, their sensitiveness diminishes and it is
necessary to raise the temperature to 58°–62° C. in order to destroy
their activity.
[Sidenote: [574]]
Bordet maintains that the cytases are very different in the various
species of animals, but that in the same species only one cytase exists.
Ehrlich and Morgenroth, on the other hand, hold that the same serum
contains several, sometimes many, different cytases. This question is
too difficult to be definitely solved at present. It appears to me very
probable that there exist, in the same species of animal, two different
cytases. One of these, the macrocytase which is found in the lymphoid
organs and in the serum of the blood, acts more particularly on animal
cells. Thanks to this substance an extract or maceration of the spleen,
omentum or lymphatic glands dissolves the red blood corpuscles more or
less readily; these extracts and macerations, however, are incapable of
destroying bacteria. When the macrophages seize the nucleated blood
corpuscles they digest them completely, not sparing even the nucleus, so
resistant to attack, but when the same phagocytes ingest such
micro-organisms as are most easily digested, such as the cholera vibrio,
their action is feeble. The vibrios, without any transformation into
granules, remain alive for some time and are destroyed and digested with
very great difficulty. The cytase of the microphages, or microcytase, is
distinguished by other properties. It destroys and digests easily many
micro-organisms, but has little or no action upon the red blood
corpuscles and other animal cells. The exudations which are rich in
macrophages, such as those of the lymphoid organs, are not at all or
only slightly bactericidal, but exhibit a solvent action on red blood
corpuscles. On the other hand, the exudations, which are composed in
great part of microphages, leave red blood corpuscles intact, but
readily destroy micro-organisms. Similar properties distinguish the bone
marrow, extracts and suspensions of which do not dissolve red
corpuscles, but attack micro-organisms. Now, we know that the bone
marrow is the principal seat of origin of the microphages.
Even after the addition of some of the specific fixative to the
microphagic exudations no solution of the red corpuscles is produced,
which demonstrates most clearly that the microcytase is really incapable
of attacking these animal cells.
We are, therefore, compelled to accept the existence of two different
cytases, of which one (the macrocytase) acts specially upon elements of
animal origin, and the other (the microcytase) acts principally on
micro-organisms. The indication of any more detailed differentiations is
impossible in the present state of our knowledge.
[Sidenote: [575]]
There are certain ferments which, during the life of the cells which
produce them, pass readily into the surrounding fluids. For instance,
sucrase can be recovered without difficulty from the culture fluid of
moulds and yeasts. The ferments of the intestinal digestion also pass
with great facility into the secreted fluids. Other soluble ferments, on
the other hand, remain very closely bound up with the cells which
manufacture them. Thus the zymase of the yeasts can only be freed from
the cells of these fungi with great difficulty, under the influence of
great pressure and under conditions which profoundly alter the cell. The
proteolytic ferment of the yeast is also very adherent to the cells of
these organisms. The fibrin-ferment, or plasmase of the white
corpuscles, is not secreted by these cells so long as they are quite
intact. But it is sufficient to subject them to unfavourable conditions
of existence to cause them to throw it out from their bodies. The
leucocytes, when removed from the animal, undergo a deterioration which
soon leads to the deposition around them of filaments of fibrin.
The cytases must also be grouped with the soluble ferments which are not
thrown off by the phagocytes so long as these remain intact. Immediately
these cells are injured, however, they allow a part of their cytases to
escape. In the blood, withdrawn from the animal, the white corpuscles
allow the plasmase to pass into the fluid, where it sets up the
coagulation of the fibrin and the formation of a clot. At the same time
these cells give up some of their cytases which communicate to the serum
its haemolytic and bactericidal properties. This fact is of the highest
importance in connection with the question of immunity. The best
demonstration of this has been furnished by a comparison of the
bactericidal power in the different parts of the body and in the body
fluids extracted from the animal.
When micro-organisms are introduced into those situations in the
refractory animal which contain pre-existent leucocytes, the leucocytes,
under the influence of the shock, undergo serious lesions, accompanied
by the throwing out of the cytases. Under these conditions the least
resistant micro-organisms (such as the cholera vibrio) exhibit
undeniable signs of deterioration: they become transformed into granules
and may even die in greater or less numbers. When, however, the
leucocytes are well protected and withstand the injection of the
micro-organisms without being profoundly altered, the extracellular
destruction of the micro-organisms does not take place. On the contrary,
a very rapid phagocytosis is produced which brings about the death and
intracellular digestion of these micro-organisms. Under these conditions
vibrios are also transformed into granules and perish, but only within
the leucocytes. The phenomena I have just mentioned are brought about in
the peritoneal cavity and in the blood vessels of refractory animals,
that is to say, in situations rich in leucocytes.
[Sidenote: [576]]
In the subcutaneous tissue, in the fluids of oedemas and in the anterior
chamber of the eye of these same refractory animals, the phenomena are
very different. As in these situations there are no pre-existing
leucocytes or their number is insignificant, the micro-organisms
introduced do not suffer serious injury; they continue to live up to the
moment when the leucocytes, having come up as the result of the
inflammatory reaction, seize them alive, kill them, and digest them
within their substance. Just as it is easy, in situations populated by
pre-existing leucocytes, to suppress the extracellular destruction of
the micro-organisms by preserving the phagocytes against injury or
phagolysis, so this same extracellular destruction is easily set up in
situations where leucocytes are absent. When, after exudations rich in
leucocytes have been injected into the subcutaneous tissue, we introduce
micro-organisms which are not very resistant, such as the cholera
vibrio, it is observed that these vibrios are destroyed outside the
cells, having first been transformed into granules.
There can be no doubt as to the conclusion to be drawn from these
various experiments. The microcytase is the substance which transforms
the vibrios into granules. It is within the microphages, when they
remain intact, that the vibrios undergo transformation. When, on the
other hand, the microphages are injured and allow the microcytase to
escape, the transformation of the vibrios into granules and their
partial destruction take place in the plasmas outside the phagocytes.
This conclusion is supported by comparative researches on the
bactericidal power of the serum and of the blood plasma outside the
animal. It is true that it is impossible to prepare a fluid which shall
in all respects be comparable to the plasma of the circulating blood.
There is, however, always a means of obtaining outside the animal a
fluid which approaches much more closely to blood plasma than does
serum. Gengou succeeded in preparing in tubes coated internally with
paraffin a fluid which coagulates very tardily, and which contains very
little fibrin-ferment. This fluid is found to be much less bactericidal
than is the blood serum of the same animal. It is, indeed, often found
to be entirely without bactericidal power, whilst the corresponding
serum is capable of destroying a large number of micro-organisms.
[Sidenote: [577]]
In the phenomena of the absorption of cells also a great number of facts
are met with which demonstrate that the macrocytase escapes from the
macrophages at the moment of their phagolysis only. For example, the
extracellular solution of the red corpuscles takes place easily in the
peritoneal fluid of animals prepared by a previous injection of the same
corpuscles. When the leucocytes of the peritoneal cavity are abandoned
to their fate, a marked phagolysis is produced and consequently a
solution of the red corpuscles in the fluid itself. When, on the other
hand, phagolysis is prevented, the macrophages remaining intact do not
allow their macrocytase to escape and the solution of the red corpuscles
takes place almost exclusively inside the phagocytes.
In certain animals the blood serum arrests the movements of their own
spermatozoa at once, whilst these remain quite motile in the animal
itself. This is due to the fact that the immobilising macrocytase is
contained within the macrophages and does not escape from them so long
as these cells remain intact. When, in such animals, their own
spermatozoa are introduced into the subcutaneous tissue, they remain
motile for a long time; when, on the contrary, the spermatozoa are
injected into the peritoneal cavity, where the leucocytes have not been
prepared, phagolysis is produced at once and the spermatozoa become
motionless immediately.
As all these data agree in demonstrating that the uninjured phagocytes
retain the cytases—which remain within them, and are not found in the
surrounding fluids,—we can readily understand the reason for the
differences between the phenomena of immunity and the bactericidal power
of the body fluids. The rat’s serum is capable of destroying a large
number of anthrax bacilli, although these rodents are certainly
susceptible to anthrax. The reason for this is that in the serum of the
rat the bacilli are destroyed by the microcytase which is set at
liberty, whilst in the body of the animal it remains enclosed within the
bodies of the living microphages. So long as these cells exhibit a
negative chemiotaxis against the anthrax bacillus, the micro-organism
remains in the plasma, where it is not interfered with. Thanks to this,
multiplication of the bacilli goes on in the body of the animal, the
micro-organism killing it after becoming generalised in the blood and in
the organs. The susceptibility of the leucocytes is, then, the cause of
the death of the rats from anthrax, the organism of these rodents being
unable to take advantage of its richness in bactericidal microcytase.
Another paradoxical fact is met with in guinea-pigs immunised against
Gamaleia’s vibrio (_Vibrio metchnikovi_). As demonstrated by von Behring
and Nissen, the blood serum of these guinea-pigs is very bactericidal
for the vibrio in question. A contact of less than an hour is quite
sufficient to destroy large numbers of the micro-organisms.
Nevertheless, when a small dose of a culture is injected subcutaneously
into these hypervaccinated guinea-pigs, the vibrios remain alive for
several days, up—indeed, to the moment when they are ingested and
destroyed by the leucocytes which come up in large numbers to the
menaced spot. This apparent contradiction is easily explained by the
fact that it is in the serum only that the vibrios encounter the
microcytase, which has escaped from the microphages at the time of the
formation of the clot and the separation of the serum.
[Sidenote: [578]]
Alongside those cases in which the serum of susceptible animals is found
to be very bactericidal, examples are not wanting where the blood and
the serum of refractory animals are entirely without this power. For
instance, the pigeon is refractory to Pfeiffer’s influenza bacillus, but
the blood of the pigeon forms the best culture medium for this
micro-organism. The dog is refractory to the anthrax bacillus, against
which the blood serum of the same animal is not at all bactericidal. The
cause of this absence of parallelism between immunity and the
bactericidal power of the serums must be sought in the difficulty with
which the cytases escape from the leucocytes, and also in the
modifications which they may undergo, once they are distributed in the
fluids.
[Sidenote: [579]]
In cases of natural immunity, the cytases rid the animal of the
micro-organisms without the slightest observable co-operation on the
part of other soluble ferments. It is impossible to settle definitely
even the question whether, in animals which enjoy this innate immunity,
there exists, alongside the microcytase, any ferments which come to its
aid. The conditions are quite otherwise in a very large number of cases
of acquired immunity. Here it is found, as a fairly general rule, that
in addition to the microcytases there exist other substances whose rôle
in the defensive action offered by the animal against micro-organisms is
very important. These substances are fixatives which co-operate in a
remarkable fashion with the bactericidal action of the cytases; but
whilst these latter injure the bacterial cell directly, the fixatives do
not interfere with its life. The bacteria, permeated by fixatives, may
even continue to reproduce themselves and, under certain conditions, to
invade the animal. The fixatives, then, are not bactericidal, but by
fixing themselves upon the micro-organisms they render them much more
susceptible to the bactericidal action of the microcytases. These latter
are further distinguished, in several other respects, from the cytases.
The fixatives must also be classed with the group of soluble ferments,
but they resist much higher temperatures than those which destroy the
cytases. Whilst the latter are quite destroyed at 55° C., the fixatives,
to be completely altered, must be heated to beyond 60° C. and even 65°
C. On the other hand, the fixatives are distinguished by a high
specificity which is never observed in the cytases. The majority of the
fixatives are incapable of fixing themselves upon more than a single
species of bacteria or upon a single class of animal cells, and only
certain of them can fix themselves upon allied species or cells, such as
the red corpuscles of several species of animals. In these cases, too,
there exists a sharp quantitative difference between the fixation on the
different formed elements. The same microcytases are, on the other hand,
able to attack all kinds of micro-organisms, and the same macrocytases
attack all kinds of animal cells.
We have seen that the cytases correspond to the zymase and to the
proteolytic diastase of the yeasts in the sense that all these soluble
ferments adhere with tenacity to the cells which produce them and
contain them. The fixatives, in this respect, approach sucrase
(invertin): these various soluble ferments pass readily into the fluids
which bathe the cells that produce them. The fixatives are found not
only in the blood serums, prepared outside the body, but also in the
blood plasma, whence they pass into the fluids of the exudations and
transudations. Whilst no cytases are found in the subcutaneous tissue,
or in the clear fluids of oedemas containing no, or almost no, cells,
fixatives are not absent from these various situations just indicated.
For this reason, when micro-organisms are introduced subcutaneously,
they are not found to be altered by the cytases, but it is easily seen
that they are permeated with fixatives. The same rule applies to the
fixatives of the animal cells. In the example we have cited, the
spermatozoa, in an animal whose serum renders these cells motionless,
remain quite motile in the epididymis and below the skin. From this fact
it may be concluded that these situations contain no free macrocytase.
It is sufficient, however, to add to these motile spermatozoa a drop of
normal serum containing macrocytase to stop their movements at once, the
fixative being well distributed in the plasma of the living animal. The
spermatozoa, then, were sensibilised by the fixative which was found in
both the epididymis and in the subcutaneous tissue.
[Sidenote: [580]]
The cytases are soluble ferments which are essentially intracellular:
the fixatives are, on the other hand, soluble ferments which are
humoral. These fixatives, however, although circulating in the plasmas,
are undoubtedly of cellular origin. This fact was first demonstrated by
Pfeiffer and Marx, who found the specific fixative of cholera vibrios in
the “haematopoietic organs,” that is to say, in the spleen, lymphatic
glands, and bone marrow, at a period when there was, as yet, none in the
blood. This fact has been extended to other examples of fixatives of
micro-organisms, and it cannot be questioned that the phagocytes produce
these soluble ferments. Under the influence of the introduction of
micro-organisms into the body, a phagocytic reaction is produced which
has, as a consequence, the digestion of these micro-organisms and the
production of corresponding fixatives. There is every reason to believe
that, in these cases, it is the microphages which, seizing and digesting
the micro-organisms, produce the fixatives.
But the macrophages are also capable of producing these adjuvant
ferments. Even in normal animals the macrophagic organs, such as the
spleen, and especially the mesenteric glands, contain fixatives which
help in the solution of the red blood corpuscles. Into this group of
facts we must also place the production by the mesenteric glands, as
well as by certain other lymphoid organs, and the leucocytes of
exudations and the blood, of enterokynase,—the soluble ferment which
aids the digestive action of trypsin. This enterokynase is also a
species of fixative; it permeates the flakes of fibrin and renders them
much more accessible to the influence of the trypsins.
The fact that the enterokynase of the intestinal digestion corresponds
in so many respects to the fixatives which act in the absorption of
formed elements in general and of micro-organisms in particular,
furnishes a further proof that the destruction of micro-organisms in the
animal is an act similar to true digestion.
[Sidenote: [581]]
Phagocytes, those elements which accomplish the absorption of
micro-organisms and of animal cells, those holders of digestive cytases,
are also the manufacturers of fixatives. Having brought about this
absorption, the phagocytes set to work to elaborate large quantities of
fixatives, although they are unable to increase the amount of cytases in
any marked degree. The fixatives, produced in abundance, can be excreted
outside the phagocytes and pass into the blood plasma, and, with it,
into the fluids of exudations and transudations. But this excretion is
not an indispensable act for the functioning of the fixatives. As these
ferments prepare the way for the digestive action of the cytases, it is
necessary only that they should be able to fix themselves on the formed
elements before the latter. It is, therefore, easy to explain cases of
acquired immunity in which no fixatives are found in the body fluids.
Such examples are not rare, and are characterised by the absence of any
protective action on the part of the blood serum. In these cases, the
fixatives, whose existence is very probable, remain lodged within the
phagocytes, just as are the cytases. Within these digestive cells the
fixatives may quite well fulfil their preparatory rôle, this being
followed immediately by the action of the cytase. The same rule may
apply also to the cases of absorption in the unprepared animal, where
fixatives are not found in the blood serum, but where they are able to
act within phagocytes.
The excretion of fixatives into the plasmas, which constitutes the rule
in cases of acquired immunity, presents an analogy with the excretion of
pepsin into the blood. This soluble ferment can and does pass habitually
from the stomach into the blood and thence into the urine, where it is
often met with. As the pepsin, which only acts in an acid medium, cannot
be utilised in the alkaline blood plasma, it is evident that its
excretion is only the consequence of a too abundant over-production.
In recent years great attention has been paid to the essential mechanism
of the action of fixatives on the formed elements on the one hand, and
on the cytases on the other. According to Ehrlich, the fixatives are
bodies intermediate between the two. In possession of two haptophore
molecular groups, they are capable of entering into chemical combination
with the micro-organisms or the animal cells on the one hand, and with
the cytases on the other. It is for this reason that Ehrlich applies to
them the name of “amboceptors” or “intermediary substances.” Based on
analogous examples in organic chemistry, Ehrlich thinks that the
fixatives serve to introduce the cytases into the cells upon which they
have to act. Bordet does not share this view and maintains that the
action of the fixatives is not a chemical action in the proper sense of
the word, but is a kind of mordanting which sensibilises the formed
elements to the fermentative action of the cytases. According to him,
the fixatives have no affinity for the cytases and in no way serve them
as intermediaries, for which reason he gives to them the name of
sensibilising substances. The question is still under discussion, but we
may hope that it will soon enter into its final phase.
[Sidenote: [582]]
According to Ehrlich’s theory, the fixatives contain no product coming
from the micro-organisms or from the animal cells upon which they are
fixed. The fixatives are, according to him, side-chains or receptors,
produced in excess and expelled into the blood plasma by the cells which
produce them. Ehrlich does not tell us to what category these cells
belong; he maintains only that these cells must be in possession of
receptors endowed with a specific affinity for certain molecular groups
of micro-organisms and of animal cells. As soon as the receptors are
saturated by these molecular groups, the cells which make use of the
former for their nutrition produce them in superabundant quantity. The
cells of animals, treated with micro-organisms and their soluble
products, or with red blood corpuscles or any other kind of element of
animal origin, acquire the property of elaborating more and more of the
corresponding receptors, a large proportion of which are expelled into
the blood plasma.
The common point between Ehrlich’s theory and the view maintained in
this work consists in the admission of a cellular property which
develops more and more in proportion to the treatment of the animal by
formed elements of all kinds. As, in acquired immunity against
micro-organisms, the fixatives are most frequently found in the body
fluids, it must be concluded that, in all these cases, the cells which
produce them have become adapted by a kind of education to manufacture
increasing quantities of fixatives. But even in those examples of
acquired immunity where fixatives are not found in the plasmas, we must
accept a modification of the cells which resist the invasion of
micro-organisms. These changes in the cellular properties constitute,
therefore, the most general, and consequently the most important,
element in acquired immunity against micro-organisms.
As already mentioned Ehrlich does not assign any position to the cells
which exhibit these modifications. It must, however, be accepted that
they belong to the category of phagocytes. Indeed, the phagocytes put
themselves into most intimate contact with the micro-organisms and
foreign animal cells, and it is in the phagocytic organs that the
fixatives are found before they are met with in the blood plasma. It may
then be concluded that, in acquired immunity against micro-organisms,
the phagocytes become adapted to elaborate the fixatives in large
quantities, of which a portion is excreted into the body fluids, as has
been shown in many examples of such immunity.
[Sidenote: [583]]
The progressive adaptation of the phagocytes in intracellular digestion
can be demonstrated by the fact that in an immunised animal the
fixatives are found more especially in the phagocytic organs. The
leucocytes which digest gelatine exhibit in an even more distinct
fashion the modification of these cells in animals which have received
several injections of gelatine. The leucocytes of exudations, when the
fluid is removed, become much more fitted to digest the gelatine than
they were at first.
A similar adaptation is also observed in intestinal digestion, which may
serve as a fresh point of comparison between the intracellular digestion
of the phagocytes and the extracellular digestion in the intestines. The
pancreas, in order to secrete its soluble ferments, adapts itself to the
nature of the food which passes into the digestive canal.
The fixatives are not the only soluble ferments which appear in large
quantities in the fluids of the immunised animal. Very often there are
found along with them substances which agglutinate the micro-organisms
in animals which have received several injections of micro-organisms of
the same or an allied species. The same fact is observed in animals
treated with animal cells. Thus the fluids of animals injected with
blood corpuscles become agglutinative for these corpuscles.
[Sidenote: [584]]
The analogy between the agglutinins and the fixatives is so great that
for some time several observers assumed them to be one and the same
substance. This can no longer be upheld, for it is clearly demonstrated
that the property of the body fluids to agglutinate micro-organisms and
animal cells is different from that which brings about their permeation
by fixatives. The agglutinins resist the same temperatures as the
fixatives; both are specific to the same degree and pass equally from
the cells which produce them into the plasmas of the blood, lymph,
exudations, and transudations. The agglutinins capable of clumping the
formed elements into masses may, under certain conditions, render their
ingestion by the phagocytes more easy. In general, however, the part
played by the agglutinins in acquired immunity must be regarded as of
little importance, and for that reason we abstain from basing any theory
of this immunity on the agglutinative property of the body fluids.
Besides fixatives and agglutinins, the fluids of an animal which has
acquired immunity very probably possess other properties which must have
a greater or less function in acquired immunity. Thus, we are often
struck by the stimulating action of these fluids on the normal animal
into which they are introduced. This stimulation is especially
manifested against the phagocytic reaction.
As, in the majority of cases of acquired immunity, the blood serum
contains fixatives in considerable proportion, and as these fixatives
aid the action of the cytases in a remarkable fashion, we can readily
understand that the introduction of such a blood serum into a normal
animal, unprepared by any vaccination, may bring about a great
resistance against the corresponding pathogenic micro-organisms. The
fixatives, injected with the serum, fix themselves with avidity upon the
micro-organisms. These organisms may become a more ready prey to the
phagocytes and be destroyed very rapidly. In particular cases, where the
injection of microbial cultures sets up a phagolysis, enough cytases are
thrown out to affect the microbes already sensibilised by the fixative.
This is followed by a refractory condition of the animal proportionate,
in general, to the amount of fixative serum that is injected. This kind
of acquired immunity, conferred by serums or certain other body fluids
rich in fixative substances, has often received the name of passive
immunity. This term is only justified in those rare cases where the
introduced serum itself contains a sufficient amount of cytases to
destroy all the micro-organisms. Most often it is the normal animal
which has to furnish this bacteriolytic ferment. Now, as in phagolysis
the quantity given off is too small, it is to the co-operation of the
holders of cytases, that is to say, to the phagocytes, that the animal
must have recourse. The phagocytes, being susceptible cells, their
co-operation can only be counted upon in cases where they exhibit a
sufficient activity. When these elements are weakened by narcotics or by
any other cause, they become incapable of intervening with efficacy and
the animal falls a victim to the pathogenic micro-organisms, in spite of
the more than sufficient amount of fixatives that was introduced.
[Sidenote: [585]]
In natural or acquired immunity, it is the resistance of the animal
against the micro-organisms which plays the principal part. The
introduction of toxins ready prepared is only done under artificial
conditions, as in laboratory experiments. Hence we see that, under
natural conditions, it is against the penetration of the micro-organisms
that the animal must be protected. So soon as these producers of poisons
can no longer maintain themselves in the immunised animal their toxic
secretions do not come into play. It is for this reason that animals
vaccinated against pathogenic micro-organisms do not suffer from
intoxication, although they are by no means insusceptible to the
microbial poisons. It is a fact of the highest importance from the point
of view of immunity in general, that the resistance offered to
micro-organisms in no way implies insusceptibility to their poisons. The
view has frequently been expressed that, in acquired immunity at least,
the animal must first acquire immunity against the microbial toxins,
after which the micro-organisms, deprived of their principal weapon,
descend to the rank of inoffensive saprophytes. Such cases may be found,
but it is none the less true that immunity against micro-organisms may
be acquired independently of that against the toxins, and that this
constitutes the general rule.
Immunity is much more readily acquired against micro-organisms than
against their toxins. Hence, antimicrobial vaccination was accomplished
by science before that against their toxins. In the early researches on
this subject antitoxic immunity appeared to be very difficult of
attainment, and it was only after the discovery made by von Behring, who
inaugurated a new path in microbiology, that better results were
obtained. Von Behring not only succeeded in immunising animals against
some of the principal microbial toxins, he demonstrated the existence of
specific antitoxins in their body fluids.
This very unexpected conception of antitoxins at once took root in
science, for it has been possible, thanks especially to the remarkable
works of Ehrlich, to extend it to toxins of non-microbial origin. We are
already acquainted with a certain number of antitoxins which, however,
are not comparable in number to the other antibodies. Amongst these, the
fixatives have many points of analogy with the antitoxins. Like them,
they are resistant to heat: they exhibit also a fairly marked
specificity, and, like the fixatives, they are distributed in the
plasmas.
[Sidenote: [586]]
In the presence of so many points of similarity with the fixatives, one
is tempted to attribute to the two categories of antibodies the same
origin. The elaboration of antitoxins by the phagocytic elements,
accumulated in the blood and disseminated in the organs, appears, in
fact, to be very probable. Certain facts bearing on the absorption of
various toxins by the leucocytes, as well as the distribution of
antitoxins in the animal body, speak in favour of this view. On the
other hand, the impossibility of attributing the elaboration of
antitoxins to cells attacked by the corresponding toxins is quite in
harmony with the same hypothesis. This hypothesis is especially
supported by the numerous facts which prove the readiness with which the
leucocytes react against all kinds of poisons, microbial or other
toxins, as well as against organic and mineral poisons, such as the
alkaloids and the arsenical combinations. However, in spite of so many
data which speak in favour of the phagocytic origin of antitoxins, it
has been impossible to support this view by rigorous facts easy of
interpretation, such as those which science possesses in support of the
phagocytic origin of fixatives.
The antitoxins have acquired a very great importance in the artificial
cure of toxo-infective diseases, the aim in these cases being to
paralyse the action of the toxins already produced by the
micro-organisms and absorbed by the diseased animal. But their function
is less in the protection against diseases where the object to be
obtained is a reaction against the micro-organisms before these are able
to inundate the animal with their toxic secretions. It is for this
reason that the immunity against toxins must, in the study of immunity,
occupy a less preponderant place than does the immunity against
micro-organisms.
As the micro-organisms placed in the refractory animal ultimately
undergo a digestion by chemical substances elaborated by the phagocytes,
so also the toxins undergo a chemical modification due to the presence
of substances in the production of which the living elements of the
animal play a large part. The direct action of antitoxins on the toxins,
so well demonstrated, especially by Ehrlich’s investigations, does not,
however, exclude the intervention of living cells, which, though
sometimes not very manifest, is in other cases very marked.
[Sidenote: [587]]
The reaction of the living elements against the microbial toxins and
their allies leads to the production, and even the over-production of
antitoxins. According to Ehrlich, these elements are the receptors, or
side-chains, which, to a certain extent, pre-exist in the cells which
are capable of elaborating the antitoxins. On entering into combination
with the toxin molecules, the side-chains, which are indispensable for
the nutrition of the cells, are reproduced in very large numbers. After
having saturated, so to speak, the productive elements of the antitoxin,
the superfluous side-chains escape from the cell and pass into the
plasmas of the body fluids. This theory may be brought into harmony with
the other theory, which maintains that certain elements of the animal,
capable of acting on the complex molecules of microbial toxins and their
allies, produce special soluble ferments, which digest the toxins whose
introduction frequently excites the hypersecretion of the ferments. Here
we have something similar to the hypersecretion, by the glands of the
stomach, of pepsin, a part of which passes into the blood in order to
escape with the urine.
According to Ehrlich’s theory, the antitoxins are only capable of
neutralising the injurious action of toxins when the former are found
dissolved in the body fluids. The same receptors which fix the toxins in
the plasmas and thus prevent them from reaching the susceptible
elements, bring about an opposite result when they are found inside the
cells. In this latter case, the receptors, owing to their great affinity
for the toxins, attract them and allow them to pass into the cells, in
this way aiding the dangerous function of the toxophore group.
This is an ingenious idea, conceived to bring into harmony a certain
number of observed facts. In the present state of our knowledge it
cannot be subjected to rigorous experimental test. Many well-established
facts, however, are not in complete accord with this hypothesis.
According to it the antitoxic immunity resides exclusively in the body
fluids; the living cells, instead of acquiring immunity, become more and
more susceptible. Under these conditions it is difficult to conceive of
an immunity against poisons of the simplest organisms; nevertheless,
this certainly exists. A plasmodium, which becomes adapted to all kinds
of toxic substances, acquires an immunity against them, and this is due
to changes taking place in the living elements; it is not the result of
modifications in the toxic fluids which bathe them. This biological
adaptation is observed in the case of physical factors which may
interfere with the life of these primitive organisms.
On the other hand, it must be accepted that the living cells of a
complicated and higher organism may also acquire immunity against
toxins. The first example of this kind was shown in relation to the red
blood corpuscles of mammals vaccinated against the toxic serum of the
eel. Whilst the body fluids of immunised rabbits become antitoxic, their
red blood corpuscles, when completely freed from the serum, in certain
cases resist the action of the eel’s serum. It must be admitted that in
this example we have an acquired immunity of the cells similar to that
met with in lower organisms.
[Sidenote: [588]]
A second example of the immunity of the red corpuscles was observed by
Ehrlich and Morgenroth in goats prepared by injections of the blood of
other individuals of the same species. In this case, according to these
writers, no co-operation by antitoxin is met with. The body fluids of
the goats do not become capable of neutralising the toxin of the
haemolytic serum, whilst the red corpuscles themselves acquire an
immunity against this toxin, an immunity entirely cellular. Ehrlich
attempted to penetrate into the essential mechanism of the resistance of
the red blood corpuscles on the supposition that these corpuscles,
instead of reproducing their receptors, as when there is production of
antitoxin, get rid of them entirely. Deprived of receptors, they can no
longer be affected by the haemolytic cytase which, as Ehrlich maintains,
only penetrates into the red corpuscles owing to the affinity of the
intermediate substance (fixative) for the receptor. This hypothesis of
the mechanism of acquired cellular immunity scarcely accords with the
hypothesis of the special function attributed to the receptors in the
nutrition of the living elements.
Cellular immunity can be most easily demonstrated in relation to the red
corpuscles of the blood, as these elements are very numerous and are
capable of being isolated and freed from the fluid in which they are
bathed. For this reason, science does not as yet possess sufficiently
exact data on the immunity of other cells in higher animals. Many facts,
however, indicate that such immunity does exist. There are, indeed,
living elements which only acquire immunity with great difficulty and
very slowly. Such are the nerve cells, elements which are specially
susceptible. Von Behring has strongly insisted on the fact that in
animals subjected to repeated injections of bacterial toxins, the nerve
centres not only do not become accustomed to their injurious action, but
even acquire a hypersusceptibility which is often very great. The
observation is perfectly accurate, but it is none the less true that
this period of exaggerated susceptibility is followed by another, during
which the susceptibility becomes less marked and ends by giving place to
a true adaptation. We are, therefore, compelled to accept the fact that
even the nerve cells are no exception to the general rule, but are able
to acquire a diminished susceptibility to a poison.
[Sidenote: [589]]
Several facts of another series confirm this conclusion. In the study of
the action of the nervous system one frequently has occasion to observe
instances of adaptation. I will cite as an example the adaptation of
animals to spinal concussion studied by Lépine[922]. By percussing the
lumbar region of rabbits and guinea-pigs we may induce in them an
immediate paraplegia. This is transitory, and lasts at most for a few
hours. The phenomenon may be reproduced several times in the same
animal. “But,” remarks Lépine, “when these experiments are continued for
several days or several weeks, striking always at the same level, we
soon observe that the resistance of the animals to the blows increases
very rapidly, and that excitations which, in normal animals, produce
paraplegias of several hours’ duration, produce no effect upon those
which have been under experiment for several days.” We have in this
example a real adaptation of the spinal region when subjected to
concussion.
Similar facts are known to everyone as an experience of daily life. We
can become habituated more or less easily to all kinds of violent
sensations. Light and very intense noises which, at first, excite
exaggerated reflex actions are ultimately perceived without setting up
the least movement. Even in the psychical sphere habit dulls painful
feelings, and it is very probable that a whole gamut of adaptation,
starting from unicellular organisms which accustom themselves to live in
an unsuitable medium, up to cultured human beings who habituate
themselves to a disbelief in human justice, will be found to rest upon
one and the same fundamental property of living matter.
[Sidenote: [590]]
Regarded from this point of view, immunity becomes a very general
phenomenon, passing far beyond the resistance offered by the animal to
infective diseases. After all is said and done, it invariably reduces
itself to that cellular susceptibility [irritability] which governs so
many of the vital phenomena in plants and in animals. It is this
susceptibility which impels the branch towards the light and the root
towards the ground, and which guides the spermatozoon towards the ovum.
From the very commencement of embryonic life the cells derived from the
segmentation of the egg exhibit a marked susceptibility. Wilhelm
Roux[923] observed that the earliest cells of the frog embryo, if they
are separated by artificial intervention, guided by their positive
chemiotaxis again come together. In the formation of the tissues
cellular susceptibility plays an important undoubted rôle. The
prolongations of the nerve cells direct themselves towards the organs of
sense or towards the muscular fibres, according to their specific
susceptibility[924]. The mother-cells of the capillary vessels are also
guided by susceptibility, when they go towards a new-formed tissue, or
when they approach one another and come together in order to form a
vascular loop.
The phenomena of the organism which bear the sharpest impress of their
physical and chemical nature, also come under the influence of cellular
“sensations.” Thus, in gastro-intestinal digestion, the secretion of the
active juice is subordinated to the control of the nerve centres and
even of the psychic centres. The sight of various kinds of food
stimulates, unconsciously, by reflex action the activity of different
digestive glands. In the same way the contraction of the contents of the
cells of a plant subjected to plasmolysis, brings about the secretion of
acid in order to augment the osmotic pressure.
Susceptibility, whose part is so great in the phenomena of immunity,
taken as a whole, is a general property of living beings, regulated by a
common law. Thus, in the chemiotaxis of the lowest unicellular
organisms, as in the movements and the osmotic reaction of plants, there
is manifested the same psycho-physical law of Weber-Fechner which
regulates our own sensations.
[Sidenote: [591]]
All cells are able, by modifying their function under the direction of
susceptibility, to adapt themselves to changes in the surrounding
conditions. All living elements are able, therefore, to acquire a
certain degree of immunity. But, amongst all the cells of the animal
body, the elements which have retained most independence—the
phagocytes—most easily and first acquire immunity to infective diseases.
These are the cells which betake themselves to situations where
micro-organisms and their poisons make their appearance, and which
manifest a reaction against them. The phagocytes of the immune organism
ingest and destroy micro-organisms and absorb toxins and other poisons.
The final act of the reaction of the phagocytes is constituted by the
chemical or chemico-physical processes concerned in the digestion of the
micro-organisms, with the help of cytases, assisted by the fixatives; in
the defence offered against poisons the phagocytes must also exert a
chemical action. Before these phenomena come into play, however, the
phagocytes manifest phenomena which are purely biological, such as the
perception of chemiotactic and other sensations, the migration towards
menaced situations, the ingestion of micro-organisms and the absorption
of toxins, and finally the secretion of substances to be utilised in
intracellular digestion.
The immunity in infective diseases presents itself, therefore, as a
section of cellular physiology, and especially as a phenomenon concerned
in the absorption of micro-organisms. This absorption being carried out
by an act of intracellular digestion, the study of immunity comes into
the chapter on digestion regarded from the general point of view.
As in the struggle of the body of the animal against infective agents
the phagocytes play the principal part, it happens that in certain
diseases the micro-organisms in order to manifest their morbific effect
must be protected from the attacks of these defensive cells. It is for
this reason that the cholera vibrio, which is not very injurious when
introduced below the skin of the human subject, becomes very formidable
when it succeeds in gaining access to the digestive canal. Incapable of
maintaining a struggle against the phagocytes, the vibrio is able to
overcome in the stomach and in the intestines without difficulty the
obstacles which it here meets with. It is for this reason that the
channel of entrance of the micro-organisms at times plays such a
prominent rôle in immunity against infective diseases.
The question is often asked whether a theoretical study of immunity is
capable of rendering service in the search for means of conferring
immunity on the animal. It must not be forgotten that theory and
practice frequently march side by side, but that sometimes they advance
without very much regard for each other. Thus the first preventive
inoculations against snake-bite, small-pox, and pleuropneumonia,
attempted by laymen were evidently made independently of any theoretical
ideas of any kind, but were guided by the purest empiricism. On the
other hand, the theoretical researches on the nature and origin of
ferments led to the discovery of vaccinations by means of
micro-organisms and microbic products which have rendered immense
services to practical medicine.
[Sidenote: [592]]
[Sidenote: [593]]
The discovery of antitoxins, so rich in practical applications, was
influenced by theoretical researches on the mechanism of immunity. Von
Behring began his important series of investigations on this subject
with the study of the immunity of rats against the anthrax bacillus. It
did not suggest itself to anyone to suppose that this question could
have the slightest immediate practical interest; nevertheless, starting
from this investigation, von Behring, after giving up the theory of the
bactericidal property of the body fluids as a cause of immunity,
advanced, step by step, to the discovery of the antitoxic power of the
serums. When a study of the properties of the blood of animals treated
with the red corpuscles of another species was commenced, no one would
have suspected that these researches would end in the discovery of new
methods for the recognition of human blood in medico-legal researches,
or in the interests of hygiene for the determination of the source of a
milk. The cellular theory of immunity is, as yet, of too recent date for
us to claim the right to expect it to have amongst its assets methods
for purely practical application. Nevertheless, it has already been
found to be of service in the investigation of problems very closely
affecting medical practice. Lord Lister, the greatest surgeon of the
nineteenth century[925], asked himself how it was that wounds could heal
“by first intention under circumstances before incomprehensible.
Complete primary union was sometimes seen to take place in wounds
treated with water-dressing, that is to say, a piece of wet lint covered
with a layer of oiled silk to keep it moist. This, though cleanly when
applied, was invariably putrid within twenty-four hours. The layer of
blood between the cut surfaces was thus exposed at the outlet of the
wound to a most potent septic focus. How was it prevented from
putrefying as it would have done under such influence if, instead of
being between divided living tissues, it had been between plates of
glass or other indifferent material?” “How were the bacteria of
putrefaction kept from propagating in the decomposable film?
Metchnikoff’s phagocytosis supplied the answer. The blood between the
lips of the wound became rapidly peopled with phagocytes which kept
guard against the putrefactive microbes and seized them as they
endeavoured to enter. If phagocytosis was ever able to cope with septic
microbes in so concentrated and intense a form, it could hardly fail to
deal effectually with them in the very mitigated condition in which they
are present in the air. We are thus strongly confirmed in our conclusion
that the atmospheric dust may safely be disregarded in our operations;
and Metchnikoff’s researches, while they have illumined the whole
pathology of infective diseases, have beautifully completed the theory
of antiseptic treatment in surgery.” (_Rep. Brit. Ass._, p. 27.)
We may even attempt to increase phagocytosis in surgical operations,
especially in those on the peritoneal cavity, by there setting up an
artificial aseptic inflammation, by means of various substances,
innocuous in themselves, which attract a large number of leucocytes. In
laboratory practice this method is in daily use for the purpose of
increasing the resistance of an animal against intraperitoneal
injections of various micro-organisms, and Durham has suggested the
extension of the same method to human medicine. Certain surgeons have
already made attempts in this direction.
The application of the cellular theory of immunity to researches on new
micro-organisms of infective diseases has already been crowned with
success. Nocard and Roux have attempted to cultivate in the animal body
the virus of the pleuropneumonia of cattle. They selected the rabbit, an
animal naturally refractory against this infection. On the supposition
that, in this immunity, the phagocytes must play an important part as
destroyers of the presumed micro-organisms, the idea suggested itself to
them to withhold the virus from their voracity. With this object they
filled sacs of collodion or of reed pith with pleuropneumonia virus, and
introduced these sacs into the peritoneal cavity of rabbits. Some time
after this operation these investigators were able to demonstrate in the
contents of the sacs impregnated by the blood fluid of rabbits, immune
animals, the development of specific micro-organisms, the smallest
discovered up to the present. By means of cultivations of this
micro-organism, obtained in suitable media, they worked out a method of
vaccinating animals which, as mentioned in Chapter xv., has already
begun to give good results in veterinary practice. This method has thus
contributed to the prevention of diseases, a branch of knowledge which
has made such great advances since medicine became an exact science
under the inspiration of the discoveries and ideas of Pasteur.
[Sidenote: [594]]
Within a very short period immunity has been placed in possession not
only of a host of medical ideas of the highest importance, but also of
effective means of combating a whole series of maladies of the most
formidable nature in man and the domestic animals. Science is far from
having said its last word, but the advances already made are amply
sufficient to dispel pessimism in so far as this has been suggested by
the fear of diseases, and the feeling that we are powerless to struggle
against them.
LIST OF AUTHORITIES QUOTED.
Abel, 443, 444, 445, 536
Abel. _See_ Loeffler
Achalme, 96
Achard and Bensaude, 264, 451
Adil Bey. _See_ Nicolle
Almquist, 178
Arloing, 264, 452
Arloing, Cornevin and Thomas, 471
Arnold, 411
Arthus, 95
Babes, 75, 348
Bach, 408, 410
Bail, 151, 185, 359
Balbiani, 13, 23, 133
Bardach, 150
Barthels, 507
Bary (de), 31, 32
Batzaroff, 411
Baumgarten, 138, 193, 521, 522, 524
Bayeux. _See_ Roger
Behring, 20, 152, 153, 205, 242, 290, 334, 335, 348, 350, 352, 367,
369, 374, 375, 378, 417, 526, 540, 561, 564, 567
Behring and Kitasato, 266, 344, 347, 354, 357, 493, 495
Behring and Kitashima, 42, 290, 368, 370, 373
Behring and Knorr, 355
Behring and Nissen, 211, 226, 526, 531
Bensaude, 439
Bensaude. _See_ Achard
Bernard, 59
Bernheim, 408
Bertrand. _See_ Phisalix
Besredka, 111, 191, 231, 263, 273, 318, 353, 390, 396
Besson, 170
Beumer and Peiper, 230
Biedl and Kraus, 44
Birch-Hirschfeld, 514
Bitter, 525
Bizzozero, 48, 177, 418, 428
Bjelooussoff, 55
Blagovestchensky, 323
Bolton, 205
Bordet, 22, 68, 79, 87, 90, 94, 95, 105, 107, 111, 112, 115, 123, 166,
179, 185, 193, 194, 196, 199, 215, 217, 223, 244, 251, 256, 257,
258, 282, 298, 302, 313, 320, 321, 535, 537
Bordet. _See_ Gengou
Bordet and Danysz, 467
Borrel, 478
Borrel. _See_ Roux, Yersin
Bouchard, 184, 232, 286, 323, 343, 427, 529
Bouchard and Charrin, 42, 528
Bourne, 327
Braun, 12
Brieger, 369
Brieger and Fränkel, 344
Briot, 109
Brücke, 66
Brunner, 45
Buchner, 87, 95, 184, 185, 188, 193, 255, 357, 362, 377, 412, 512, 527,
528, 530, 539, 540
Cahanescu, 430
Calmette, 334, 339, 345, 346, 347, 348, 358, 365, 386, 389, 395, 425,
489
Calmette. _See_ Yersin
Calmette and Deléarde, 365
Calmette and Salimbeni, 491
Camus and Gley, 110, 121, 360
Cantacuzène, 224, 225, 306
Castle. _See_ Davenport
Cattani, 446
Cayley, 484
Celakovsky, 30
Celli, 278
Centanni, 446
Chamberland, 470
Chamberland. _See_ Pasteur, Roux
Chantemesse, 259
Chantemesse and Widal, 230, 267, 319, 437
Chapeaux, 55, 56
Charrin, 232, 286, 287, 427, 428, 541
Charrin. _See_ Bouchard
Charrin and Gamaleia, 290, 343
Charrin and Gley, 446
Charrin and Lefèvre, 419
Charrin and Magnin, 427
Charrin and Roger, 232, 256
Chatenay, 393
Chauveau, 289, 446, 455, 511, 512
Chépowalnikoff, 59
Cherry. _See_ Martin
Cienkowski, 446
Cobbett, 205
Cohn, 23
Colombot. _See_ Sabrazès
Cornevin, 452
Cornevin. _See_ Arloing
Couch, 53
Courmont, 400
Courmont. _See_ Nicolas
Courmont and Doyon, 330, 386, 394
Curtis, 172
Czaplewski, 146, 147
Dallinger, 26
Danysz, 21, 25
Danysz. _See_ Bordet
Daremberg, 87
Darwin, 8
Davenport and Castle, 27
Davenport and Neal, 24
Decroly and Rousse, 396
Deléarde. _See_ Calmette
Delezenne, 61, 96, 98, 107, 116
Delezenne and Froin, 66
Delius and Kolle, 277
Dembinski, 147
Denys, 533
Denys and Havet, 151, 185
Denys and Kaisin, 151
Denys and Leclef, 243, 246, 283, 312
Denys and Marchand, 313
Denys and van de Velde, 359
Deutsch, 107, 293, 294, 537
Dienert, 26
Dieudonné, 139, 143, 147
Dinkelspiel. _See_ Nuttall
Doederlein, 429
Dönitz, 391
Dominici, 78
Dominici. _See_ Gilbert
Doyon. _See_ Courmont
Dreyer, 350
Duclaux, 26
Dujardin-Beaumetz, 478
Dungern (von), 91, 109, 123, 324
Durham, 256, 261, 569
Dzierzgowsky, 448, 449
Effront, 26
Ehrlich, 114, 115, 344, 346, 349, 356, 360, 361, 365, 378, 391, 392,
420, 449, 562, 563
Ehrlich and Hübener, 446, 452
Ehrlich and Lazarus, 76
Ehrlich and Morgenroth, 88, 89, 92, 95, 104, 114, 116, 124, 193, 194,
199, 268, 537, 538, 563
Ehrlich, Kossel and Wassermann, 496
Ehrlich and Wassermann, 356
Elmassian. _See_ Morax
Emden (van), 264
Emmerich, 237, 322, 527
Emmerich and di Mattei, 236, 527
Emmerich and Löw, 254
Emmerich and Mastbaum, 475
Ermengem, 420, 491
Errera, 39
Escherich. _See_ Klemensiewicz
Faber (Knud), 344
Fahrenholtz, 138
Fehleisen, 434
Fermi and Pernossi, 109
Ferran, 480
Fischer, 193, 213, 253
Fischl and Wunschheim, 445
Fleck, 413
Flügge, 43, 184, 525, 540
Fodor, 184, 525
Foerster, 380
Fontana, 333
Forssmann, 565
Frank, 35, 154, 542
Fränkel, 344, 347, 499, 534
Fränkel. _See_ Brieger
Fränkel and Sobernheim, 268
Frantzius, 425
Fraser, 345, 425
Frédéricq, 55, 57
Freudenreich, 323
Freund, Grosz and Jelinek, 365
Froin. _See_ Delezenne
Funck, 267, 319, 320, 456
Galeotti. _See_ Lustig
Gamaleia, 419
Gamaleia. _See_ Charrin
Garnier, 220, 304
Gaule, 515
Gautier, 400
Gengou, 19, 20, 146, 151, 157, 185, 190, 203, 242, 252, 255, 260, 264,
308, 543
Gengou and Bordet, 190
Geret. _See_ Hahn
Gheorghiewsky, 210, 234, 236, 261, 269, 301, 307, 359
Gibier, 137
Giessler, 37
Gilbert and Dominici, 424
Gilkinet, 172
Gley. _See_ Camus, Charrin
Glogner, 434
Goldschmidt, 411
Gottstein, 499
Gramatschikoff, 412
Grancher. _See_ Pasteur
Grawitz, 513, 515
Griffon. _See_ Landouzy
Grosz. _See_ Freund
Gruber, 224, 256, 262, 542
Gscheidlen. _See_ Traube
Guarnieri, 455
Guinon. _See_ Voisin
Günther, 541
Haeckel, 517
Haffkine, 480, 486–488
Hafkine, 17
Hahn, 188, 190
Hahn and Geret, 197
Hankin, 156, 187
Hardy. _See_ Kanthack
Harnack, 337
Häser, 507
Havet. _See_ Denys
Hayem, 47, 514
Hegeler, 196
Herbst, 565
Héricourt. _See_ Richet
Herzen, 62
Hess, 144, 149, 524
Hewlett. _See_ Thomson
Heymans. _See_ Lang
Heymans and Masoin, 396
Hildebrandt, 109, 119, 412
Himmel, 182
Hippocrates, 342
Hirsch and Mehring, 64
Hoffmann and Recklinghausen, 46
Horvath, 337
Hübener. _See_ Ehrlich
Hudalo, 436
Hueppe, 254
Hugenschmidt, 415
Issaeff, 219, 262, 287, 318, 320, 441
Issaeff. _See_ Pfeiffer
Jakowski, 42
Jeanselme, 411
Jelinek. _See_ Freund
Jenner, 507
Jetter, 193
Jona, 172
Joubert. _See_ Pasteur
Kaisin. _See_ Denys
Kanthack, 360, 542
Kanthack and Hardy, 185
Karlinsky, 134, 260