One of the features differentiating the pneumonic process in influenza
from the usual types of inflammation of the lung, is diffuseness (90).
In the early cases especially, or in cases which terminate fatally at an
early period, both lungs are often involved, and, on histological
examination, only a small portion of the pulmonary parenchyma is found
unaffected. The exudate, largely acellular, presents serum as its most
conspicuous feature. The picture is one of a patchy pneumonia with
intermediary areas of what might be called edema, although fibrin is
often demonstrable in the coagulated, albuminous material. So little
attention has been paid to earlier stages of the usual types of
pneumonia, for example lobar, that it is impossible to say whether or
not diffuse involvement of the pulmonary parenchyma initiates the
process which later becomes localized in one or more lobes. The initial
edema of influenzal pneumonia is the expression of a widespread
irritation. If the injury has not extended deeply, the edema may
disappear within a relatively short time, and exfoliated lining cells
fill the alveoli; those remaining in their normal position are
frequently in process of division (Fig. XLVII). It is conceivable—and
the view has already been announced—that this edema is a disseminating
factor and perhaps responsible for the diffuseness of the pneumonic
process which may follow. If the fluid is simply a serous exudate, it
may play no essential rôle in the severe acute symptoms manifested by
these patients, for it has been shown, both in the experimental lesions
induced by pulmonary irritating gases and by pulmonary irrigation
through which extensive artificial edema of the lungs may be attained,
that the presence of fluid in the lung in itself is not harmful (161).