## Histopathological Features of Tuberculosis **Key Point:** The caseating (tuberculous) granuloma is the pathognomonic hallmark of tuberculosis and distinguishes it from other granulomatous diseases. ### Structure of Caseating Granuloma The classic tuberculous granuloma has a characteristic layered architecture: 1. **Central caseous necrosis** — acellular, cheese-like debris with fragmented nuclei and cellular remnants 2. **Epithelioid histiocytes** — activated macrophages with elongated nuclei arranged in a palisade around the necrotic center 3. **Langhans giant cells** — multinucleated giant cells formed by fusion of epithelioid cells, with nuclei arranged in a peripheral horseshoe or crescent pattern 4. **Outer rim of lymphocytes and fibroblasts** — chronic inflammatory infiltrate that may undergo fibrosis **High-Yield:** The presence of **caseous (caseating) necrosis** is the key distinguishing feature from other granulomatous diseases like sarcoidosis (non-caseating), fungal infections, and foreign body reactions. **Mnemonic:** **CHELF** — Caseous necrosis, Histiocytes (epithelioid), Epithelioid cells, Langhans giant cells, Fibroblasts (outer rim) ### Comparison with Other Granulomas | Feature | TB (Caseating) | Sarcoidosis (Non-caseating) | Fungal | Foreign Body | | --- | --- | --- | --- | --- | | Central necrosis | **Caseous (yes)** | Absent | Variable | Absent | | Epithelioid cells | Present | Present | Present | Absent/minimal | | Langhans giant cells | Present | Present | Present | Foreign body type | | Acid-fast bacilli | Positive (Ziehl-Neelsen) | Negative | Positive (special stains) | Negative | **Clinical Pearl:** While Langhans giant cells are seen in both TB and sarcoidosis, the **caseating necrosis** is virtually pathognomonic for TB in the appropriate clinical context. Non-caseating granulomas suggest sarcoidosis, fungal disease, or berylliosis.
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