Respiratory System Disorders and Therapy From a New - Louis Bolk ...

2.3.2. Pathophysiology of Bacterial Pneumonia
Macroscopically, a uniform, sharply demarcated area of the lung parenchyma with an
infectious fluid exudate is found, that may progress to involvement of an entire lobe. It is
harder to separate the airway walls from the airway space in this area. The typical shape
of the lung is less distinct. The air in this region is displaced and the lung — or a part of
it — is converted to an airless, watery organ. This part of the lung will sink when put into
water. The infected area usually abuts the visceral pleura between the lobes or over the
convex outer perimeter of the lung. The margin that does not lie next to the pleura is
mostly well defined and the adjacent parenchyma is not involved. Larger bronchi within
the infected area often still contain air, which creates the so-called air bronchogram on
X-ray (fig. 2.2.).
Microscopically there is alveolar capillary dilatation and alveolar edema.
The exudate is initially relatively low in cells but contains numerous bacteria (Streptococcus
pneumoniae or pneumococcus). Inflammatory cells such as leukocytes are sparse at
first, but abundant later on, the microorganisms disappear. At a later stage, leukocytes
disappear and macrophages take their place. There is no tissue necrosis. If the patient
survives this infection, there is, normally speaking, a complete resolution and resorption of
the exudate and a recovery of the histology of the lung tissue. Occasionally, the fibrinopurulent
exudate results in a so-called hepatisation of the affected area. At this stage, the
lung tissue becomes fibrinous (fig. 2.2.).
Blood Serum Changes in Pneumonia
In the serum of the patient with pneumonia in the acute phase, the alternative
complement pathway is activated. There is then an increase in type-specific
antibodies against a capsular mucopolysaccharide and an aspecific protein is
present in the serum: C-reactive protein (CRP). There is usually a leukocytosis
with a shift to the left and the appearance of immature leukocytes.