## Histopathology of Caseating Granuloma in TB ### Characteristic Features of TB Granuloma **Key Point:** The caseating granuloma is the hallmark of pulmonary tuberculosis and consists of four distinct zones from centre to periphery. ### Zones of a Caseating Granuloma (Centre to Periphery) | Zone | Composition | Characteristics | |------|-------------|------------------| | **Central** | Caseous necrosis | Acellular, amorphous, cheese-like debris; eosinophilic on H&E | | **Second** | Epithelioid histiocytes | Activated macrophages with elongated nuclei; form palisade pattern | | **Third** | Langhans giant cells | Multinucleated cells with peripherally arranged nuclei in horseshoe pattern | | **Outer** | Lymphocytes & fibroblasts | T cells predominate; fibroblasts initiate fibrosis and healing | ### Why Neutrophils Are NOT Present **High-Yield:** The central caseous necrosis in TB is **acellular** — it contains neither neutrophils nor intact cells. This distinguishes TB from pyogenic infections (like staphylococcal abscess) which show abundant neutrophilic infiltration with pus formation. **Clinical Pearl:** The absence of neutrophils in the centre of a TB granuloma is a key diagnostic feature. If you see a granuloma with central neutrophilic abscess, think of: - Cat-scratch disease (Bartonella henselae) - Tularemia - Brucellosis - NOT tuberculosis **Mnemonic:** **ELICIT** — Epithelioid cells, Langhans giants, Lymphocytes, Isolated caseous necrosis (no neutrophils), Central acellular zone, TB pathology ### Why the Other Options Are Correct - **Central caseous necrosis** ✓ — defines "caseating" granuloma - **Langhans giant cells** ✓ — formed by fusion of epithelioid histiocytes; nuclei arranged peripherally - **Outer lymphocytes & fibroblasts** ✓ — provide immune response and structural support [cite:Robbins 10e Ch 8] [cite:Park 26e Ch 5]
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