Pathogenesis and General Morphology
The development of the disease begins when the pathogen primarily attacks the elements of the alveolar wall: type I and type II pneumocytes, as well as capillary endothelium. Acute inflammation erupts in this zone, rapidly engaging immunopathological mechanisms—both immediate and delayed-type hypersensitivity reactions.
The morphological picture of interstitial pneumonia is stereotyped and includes the following features:
- Damage to the alveolar epithelium with subsequent regeneration.
- Marked hyperemia of alveolar capillaries.
- Inflammatory infiltration of the interstitial tissue.
- Accumulation of proteinaceous fluid in the alveolar lumina.
- Formation of hyaline membranes (very common).
- Presence of macrophages and polymorphonuclear leukocytes, occasionally with characteristic cellular inclusions.
- In most cases, the disease ends in complete recovery. However, inflammation frequently progresses to interstitial fibrosis, and in severe progressive cases, fatal outcome is possible due to cor pulmonale and heart failure.
Viral and Mycoplasmal Infections
Viral pneumonias are most often caused by influenza viruses (the H1N1 strain poses a particular danger), parainfluenza, adenoviruses, and pathogens of childhood infections. The process often acquires a viral-bacterial character. A lympho-histiocytic infiltrate in the interstitium is typical for viral etiology, reflecting a cell-mediated immune response. Specific markers include intracellular inclusions (in cytomegalovirus and adenoviral infections) and the formation of multinucleated giant cells (in measles). The diagnosis is confirmed by immunofluorescence.
Mycoplasmal pneumonia is caused by Mycoplasma pneumoniae. It accounts for up to 15–25% of all community-acquired pneumonias, especially among children, adolescents, and individuals over 45 years of age. The process is usually unilateral and localized. Microscopy reveals a picture of acute interstitial pneumonia combined with bronchiolitis and a characteristic mononuclear infiltrate. An indirect sign is the detection of PAS-positive inclusions in macrophages (using Romanovsky-Giemsa stain and PAS reaction). A definitive diagnosis is established using immunohistochemistry (IHC).
Pneumocystis Pneumonia and Legionnaires' Disease
Pneumocystis pneumonia (pathogen — Pneumocystis carinii) is a marker disease for immunosuppressed individuals. In HIV-infected patients, it occurs in ~75% of cases. Clinically, it manifests as severe respiratory failure against the background of diffuse bilateral involvement. Microscopy reveals diffuse infiltration of the interalveolar septa. Foamy eosinophilic masses (cysts) accumulate in the alveolar lumina, yielding a positive PAS reaction. Basophilic sporozoites can be found inside the cysts. The gold standard for diagnosis is specific Grocott methenamine silver staining.
Legionnaires' disease is caused by the bacterium L. pneumophila. It was first described in 1976 following an outbreak in Philadelphia (182 cases, 29 deaths). The disease manifests with headache, myalgia, and a dry cough. A distinct feature of the pathogen is that it does not stain with Gram stain and is capable of endocytobiosis—surviving inside phagocytes. Antibodies do not destroy the bacterium but merely enhance its uptake by macrophages.