Pathological diversity of pulmonary sarcoidosis.

Sarcoidosis is a multisystem disease characterized histologically by the presence of non-caseating epithelioid cell granulomas. Although the etiology of sarcoidosis remains uncertain, Cutibacterium acnes (formerly known as Propionibacterium acnes) and mycobacteria are considered putative pathogenic antigens in sarcoidosis. Sarcoidosis must be differentiated from a variety of granulomatous diseases of the lung. The granulomas in pulmonary sarcoidosis are frequently located in the broncho-vascular bundles, interlobular septa, and alveoli, and there is frequent vascular involvement in the form of granulomatous angiitis. While most granulomas may spontaneously or with treatment disappear, some along the broncho-vascular bundles and interlobular septa often become fibrotic. In the chronic fibrotic stage, the upper lobes of the lungs become scarred and contracted, with fibrous bands seen between the hilar regions and pleura. Cystic changes and, occasionally, honeycombing are seen in the lung. Granulomatous vascular involvement is frequently seen in the lung. The microangiopathy of sarcoidosis is characterized by endothelial injury and multilamellation of the basement membrane of microvessels and has been observed electron microscopically in the heart, lung, eye, and skin specimens. The pathogenetic correlation between granulomas and microangiopathy remains to be resolved.