## Distinguishing Tuberculoid from Lepromatous Leprosy ### Immunological Basis **Key Point:** The lepromin test (Mitsuda test) is the gold standard discriminator between the two polar forms of leprosy, reflecting the host's cell-mediated immunity status. ### Comparison Table: Tuberculoid vs Lepromatous Leprosy | Feature | Tuberculoid | Lepromatous | | --- | --- | --- | | **Lepromin Test** | Positive (strong DTH) | Negative (absent DTH) | | **Acid-Fast Bacilli** | Few or absent (1–5 per field) | Numerous (>100 per field) | | **Bacterial Load** | Low (paucibacillary) | High (multibacillary) | | **Lesion Distribution** | Asymmetrical, localized | Symmetrical, widespread | | **Nerve Involvement** | Early, asymmetrical thickening | Late, symmetrical thickening | | **Cell-Mediated Immunity** | Strong | Absent | | **Skin Smear** | Negative or 1–2 bacilli | Positive, numerous bacilli | ### Clinical Pearls **High-Yield:** A positive lepromin test (papule ≥5 mm at 3–4 weeks) indicates tuberculoid leprosy and reflects intact Th1-mediated cellular immunity. A negative test indicates lepromatous leprosy with impaired cell-mediated response. **Clinical Pearl:** Tuberculoid leprosy is the immunologically competent form (few bacilli, strong immunity), while lepromatous leprosy is the immunologically incompetent form (many bacilli, absent immunity). **Mnemonic:** **TL = TH1** (Tuberculoid Leprosy = Type 1 Helper cell response = strong immunity). **LL = Th2** (Lepromatous Leprosy = Type 2 Helper cell response = weak immunity). ### Why Lepromin Test is the Best Discriminator The lepromin test directly assesses the host's delayed-type hypersensitivity (DTH) response to *M. leprae* antigens. This reflects the fundamental immunological difference between the two forms and is more specific than bacterial load alone, which can vary within the spectrum of borderline cases. [cite:Park 26e Ch 31]
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