Fig. 1

Representative histological features of acute fibrinous organizing pneumonia (AFOP). a The surgical lung biopsy specimen showed diffuse filling of alveolar spaces by fibrin balls associated with organizing pneumonia, interstitial lymphocytes and myxoid interstitial expansion (hematoxylin and eosin [H&E] stain, × 100). b The high power-view of the specimen showed intra-alveolar fibrin aggregates (arrow) which are characteristic findings of AFOP. Interstitial lymphoplasma cell infiltration, fibroblast proliferation with organization, and enlarged type II pneumocytes are present. Hyaline membranes, significant neutrophils or eosinophils are absent. (hematoxylin and eosin [H&E] stain, × 250)