Tuberculosis (TB): Pathological Essentials
Tuberculosis, caused by Mycobacterium tuberculosis, is a chronic granulomatous disease. Pathological understanding focuses on the immune response, which is fundamentally a Type IV hypersensitivity reaction.
1. Primary vs. Secondary Tuberculosis
| Feature | Primary TB | Secondary TB |
|---|---|---|
| Site | Ghon focus (mid-lower lobe). | Apex of lung (high O2). |
| Host Status | Naïve host. | Reactivation/Re-exposure. |
| Pathology | Ghon complex (Primary focus + lymphadenopathy). | Cavitation, hemoptysis. |
2. Key Pathological Features
- Caseous Necrosis: The characteristic “cheese-like” appearance of the necrotic center within granulomas.
- Granulomas: Consist of epithelioid histiocytes (activated macrophages), Langhans giant cells (fused macrophages), and a surrounding rim of lymphocytes.
- Miliary TB: Widespread dissemination via the bloodstream, leading to tiny, seed-like lesions in multiple organs (liver, spleen, bone marrow).
3. Exam Must-Knows
Pathology Pearl: The apex of the lung is the preferred site for secondary TB due to high oxygen tension, which favors the aerobic growth of M. tuberculosis. Always look for “night sweats,” “weight loss,” and “hemoptysis” in the vignette.
- Pott’s Disease: TB of the spine, often causing vertebral collapse and kyphosis.
- Diagnosis: Ziehl-Neelsen stain (acid-fast bacilli).