## Classification of Leprosy by Frequency **Key Point:** Borderline tuberculoid (BT) leprosy is the most common form of leprosy encountered in clinical practice and epidemiological surveys in endemic regions, accounting for approximately 40–50% of all leprosy cases. ### Ridley-Jopling Classification Frequency Distribution | Leprosy Type | Frequency (%) | Bacillary Load | Immune Response | |---|---|---|---| | **Borderline Tuberculoid** | **40–50** | **Few** | **Strong cell-mediated** | | Tuberculoid | 10–15 | Scanty | Very strong | | Borderline | 15–20 | Moderate | Intermediate | | Borderline Lepromatous | 10–15 | Numerous | Weak | | Lepromatous | 5–10 | Abundant | Very weak | | Indeterminate | 5–10 | Variable | Evolving | ### Why Borderline Tuberculoid Is Most Common 1. **Intermediate immune response:** Most infected individuals mount a partial but effective cell-mediated immune response, placing them in the borderline spectrum rather than at either pole. 2. **Natural history:** Indeterminate leprosy (earliest form) typically progresses toward the borderline tuberculoid end in immunocompetent hosts. 3. **Epidemiological data:** Population-based surveys in India and endemic countries consistently show BT as the modal presentation. **High-Yield:** Tuberculoid leprosy (at the immune-strong pole) is actually *less* common than borderline tuberculoid, despite being the "classic" teaching example. Do not confuse the two. **Clinical Pearl:** Borderline tuberculoid presents with few well-demarcated lesions, asymmetric distribution, and early nerve involvement—exactly what this patient likely has. **Mnemonic:** **BTBL** — *Borderline Tuberculoid is the Big Loader* (most common). Tuberculoid is the *Tiny* loader (few bacilli, rare).
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