Acute fibrinous and organizing

Beasley MB, Franks TJ, Galvin JR, et al. Acute fibrinous and organizing pneumonia a histologic pattern of lung injury and possible variant of diffuse alveolar damage. Arch Pathol Lab Med 2002 126(9) 1064-1070. 

The histologic features of acute interstitial pneumonitis (AIP) are the various phases of DAD. Initially, an increase of capillary leukocytes including neutrophils, macrophages, and lymphocytes are seen. Within a day, interstitial edema becomes more evident and type I pneumocytes undergo type II metaplasia and become swollen. As the alveolar wall becomes necrotic, fibrin and cellular residue forms a layer of eosinophiUc material that lines the alveolar wall, the so-called hyaline membrane. Hyaline membranes are the defining feature of DAD. As the disease process progresses, polyps of organizing pneumonia develop and tend to replace the hyaline membranes (Fig. 4) (3,27,28). 

Pathologic examination of the acute lesion shows extensive mucosal edema and inflammatory cell exudation. The delayed lesion shows the histologic appearance of bronchiolitis obliterans small bronchi and bronchioles contain an inflammatory exudate that tends to undergo fibrinous organization, eventually obliterating the lumen. 

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