granulomatous inflammation

Granulomatous inflammation is a distinct pattern of chronic inflammation triggered by persistent, poorly degradable antigens or T-cell mediated hypersensitivity (Type IV). Pathophysiology involves macrophages presenting antigen to CD4+ T-helper cells (Th1), which release IFN-γ, activating macrophages and transforming them into epithelioid histiocytes. These cells aggregate and may fuse to form multinucleated giant cells (Langhans giant cells with peripheral nuclei, or foreign-body giant cells with scattered nuclei). TNF-α is critical for granuloma maintenance—this is why anti-TNF therapies (e.g., infliximab) risk reactivating latent TB.

Two main types exist: (1) Caseating granulomas—central necrosis appearing as amorphous eosinophilic debris, classically seen in TB (Mycobacterium tuberculosis) and some fungal infections (Histoplasma). (2) Non-caseating granulomas—lack central necrosis, seen in sarcoidosis, Crohn disease, berylliosis, and foreign body reactions (e.g., talc, suture material).

Key associations for boards: - TB: caseating granulomas, often with Langhans giant cells, acid-fast bacilli - Sarcoidosis: non-caseating granulomas, elevated ACE levels, hypercalcemia (due to activated macrophages producing 1-alpha-hydroxylase, increasing vitamin D) - Crohn disease: non-caseating granulomas in GI tract, transmural inflammation - Fungal infections (Histoplasma, Coccidioides): granulomas, often in immunocompromised patients - Foreign body granulomas: giant cells surround inert material, no necrosis - Wegener's (GPA): necrotizing granulomas with vasculitis

Clinically, granulomas represent a protective but potentially damaging response—they sequester pathogens but can cause fibrosis, organ dysfunction (e.g., pulmonary fibrosis in sarcoidosis), and are diagnostically important on biopsy. Immunosuppression (e.g., HIV, anti-TNF drugs) impairs granuloma formation, increasing risk of disseminated infection.