Sarcoidosis is a systemic granulomatous disease of unknown etiology characterized by non-caseating granulomas in affected organs. The lungs are involved in > 90% of cases. The pathogenesis involves an exaggerated Th1 immune response to an unidentified antigen: macrophages present the antigen to CD4+ T cells, which release IFN-gamma and IL-2, driving granuloma formation. Granulomas consist of tightly clustered epithelioid macrophages and multinucleated giant cells surrounded by a rim of lymphocytes. Unlike TB granulomas, sarcoid granulomas are NON-caseating (no central necrosis). Staging is radiographic: Stage 0 (normal CXR), Stage I (bilateral hilar lymphadenopathy — BHL — alone), Stage II (BHL + pulmonary infiltrates), Stage III (pulmonary infiltrates without BHL), Stage IV (pulmonary fibrosis). Spontaneous remission occurs in 60-80% of Stage I, 50-60% of Stage II, but only 30% of Stage III, making staging important for prognosis and treatment decisions.