Interstitial pneumonia; a pathologic concept.

Interstitial pneumonias present a pulmonary inflammatory reaction pattern that is common to a wide variety of noxious agents and disease processes. These include viruses, Rickettsiae, hypersensitivity reactions, the "collagen disease," x-irradiation, uremia, certain chemicals, early histoplasmosis, toxoplasmosis and possibly unknown substances and diseases. Interstitial pneumonia is characterized histologically by interalveolar exudate, hyaline membranes, epithelialization of alveolar walls and lack of polymorphonuclear response. Histologic features that aid in differentiating among cases of varying etiology are few. It is postulated that the basic pathogenic mechanism of the interstitial inflammatory reaction is primary damage to pulmonary capillary endothelium that brings about increased capillary permeability.