Diffuse alveolar damage, clinically referred to as adult respiratory distress syndrome (ARDS), is characterized by diffuse damage to the alveolar/capillary barrier, which may result from diverse acute conditions. The four most frequent causes are trauma, sepsis, shock, and gastric aspiration. The pathogenesis is not entirely clear, but influx of neutrophils and release of cytokines, eicosanoids, and free radicals seem to be crucial in promoting alveolar damage. The most characteristic histopathologic hallmark of diffuse alveolar damage is formation of hyaline membranes within the alveolar cavities. These consist of proteinaceous material of plasma origin and necrotic debris from desquamated epithelium. The condition has a 60% moality and manifests with acute respiratory failure.Also Know:The pathologic features of asthma are relatively nonspecific and are similar, for example, to those of chronic bronchitis, including chronic inflammatory infiltration, hyperplasia of mucous glands, and hyperophy of smooth muscle. The presence of numerous eosinophils, however, is more characteristic of asthma. Bacterial pneumonia is characterized by intra-alveolar exudation of neutrophils, fibrin, and erythrocytes. Bacteria are also present within the alveolar cavities. Desquamative interstitial pneumonitis (DIP) is a form of interstitial disease referred to as idiopathic pulmonary fibrosis. In contrast to usual interstitial pneumonitis (another form of idiopathic pulmonary fibrosis), DIP is more responsive to steroid treatment. Histopathologically, DIP leads to hyperplasia of pneumocytes and accumulation of histiocytes that fill the alveolar cavities. Ref: Levy B.D., Choi A.M. (2012). Chapter 268. Acute Respiratory Distress Syndrome. In D.L. Longo, A.S. Fauci, D.L. Kasper, S.L. Hauser, J.L. Jameson, J. Loscalzo (Eds), Harrison's Principles of Internal Medicine, 18e.