## Histological Hallmark of Tuberculosis **Key Point:** The pathognomonic lesion of tuberculosis is the **caseating granuloma** (also called tubercle), characterized by central caseous (cheese-like) necrosis surrounded by epithelioid macrophages and Langhans giant cells. ### Structural Components | Component | Characteristics | |-----------|------------------| | **Central necrosis** | Acellular, amorphous, eosinophilic caseous material (pathognomonic) | | **Epithelioid macrophages** | Activated macrophages with elongated nuclei, arranged in palisade | | **Langhans giant cells** | Multinucleated giant cells with nuclei arranged in a horseshoe pattern at the periphery | | **Outer zone** | Lymphocytes, fibroblasts, and collagen forming a fibrous capsule | **High-Yield:** The presence of **caseous necrosis** (not suppurative/liquefactive) is the key distinguishing feature of TB granulomas from other granulomatous diseases like sarcoidosis (which are non-caseating). **Clinical Pearl:** Acid-fast bacilli (AFB) are typically found at the periphery of the caseous center, where macrophages are still viable and contain organisms. The necrotic center is relatively acellular and organism-poor. **Mnemonic:** **CELT** — **C**aseating, **E**pithelioid macrophages, **L**anghans giant cells, **T**uberculosis. ### Why Caseation Occurs Caseation is thought to result from: 1. Hypersensitivity reaction to mycobacterial antigens 2. Tumor necrosis factor (TNF)-α-mediated macrophage activation 3. Mycobacterial lipids triggering tissue-damaging immune response [cite:Robbins 10e Ch 8]
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