This reaction is the characteristic feature of the entity bronchiolitis obliterans with organizing pneumonia (Fig 1, center), but is a nonspecific response occurring when there is fibrinous exudation in the proximal portion of the acinus and probably also requiring relatively good preservation of the basal lamina.
In idiopathic pulmonary fibrosis, with its more smoldering progressive natural history, increased collagen synthesis is found by immunohistochemistry in the majority of biopsies, despite the fact that biochemical techniques fail to detect it. This is in part because the synthesis is extraordinarily focal, occurring in scattered small collections of fibroblasts (Fig 1, bottom). Using basal lamina as a marker, these “fibroblastic foci” are intra-alveolar, indicating that they too arise by organization of intra-alveolar exudate.
Collapse of alveoli is a well-recognized part of the pathophysiology of acute lung injury, but its importance to permanent derangement of lung architecture has only recendy been emphasized. In acutely damaged lung, whether in experimental hyperoxia, paraquat or bleomycin toxicity, or human ARDS, surfactant function is compromised by the influx of plasma components and damage to the epithelium. Alveolar collapse can result.