## Tuberculosis Granuloma Structure **Key Point:** The hallmark of TB granuloma is **caseous (caseating) necrosis**, NOT suppurative necrosis. This distinction is pathognomonic for TB and differentiates it from other granulomatous diseases. ### Characteristic Features of TB Granulomas | Feature | Description | Significance | |---------|-------------|-------------| | **Caseous necrosis** | Acellular, amorphous, cheese-like central core | Pathognomonic for TB; appears as "ghost" on H&E | | **Epithelioid histiocytes** | Activated macrophages with elongated nuclei | Central component; produce cytokines | | **Langhans giant cells** | Multinucleated cells with peripheral horseshoe-arranged nuclei | Characteristic but not specific to TB | | **Lymphocytic rim** | T cells and B cells surrounding the granuloma | Immune containment layer | | **Fibroblasts** | Outer fibrous capsule | Encapsulation and healing | **High-Yield:** Caseous necrosis is the defining feature—it is acellular and appears as a homogeneous, eosinophilic, amorphous mass. Suppurative (purulent) necrosis with polymorphonuclear leukocytes is seen in bacterial infections like abscess formation, NOT in TB. ### Why Caseous, Not Suppurative? 1. TB bacilli are intracellular pathogens; they evoke a **cell-mediated immune response** (Type IV hypersensitivity). 2. The immune response leads to **apoptosis and necrosis of epithelioid cells and lymphocytes**, creating acellular debris. 3. Suppurative necrosis occurs in acute pyogenic infections where neutrophils accumulate and release enzymes. **Clinical Pearl:** The presence of caseous necrosis in a granuloma is so characteristic of TB that its absence should prompt consideration of other diagnoses (sarcoidosis, fungal infections, foreign body reactions). **Warning:** Do not confuse caseous necrosis (TB) with suppurative necrosis (pyogenic bacteria) or fibrinoid necrosis (vasculitis).
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