## Type 2 Lepra Reaction (Erythema Nodosum Leprosum) **Key Point:** Type 2 lepra reaction is an immune complex-mediated (Type III hypersensitivity) reaction, NOT a delayed-type hypersensitivity. ### Immunological Mechanism | Feature | Type 1 Reaction | Type 2 Reaction (ENL) | |---------|-----------------|----------------------| | **Hypersensitivity type** | Type IV (delayed) | Type III (immune complex) | | **Timing** | Before, during, or after treatment | During or after treatment (usually) | | **Leprosy forms affected** | Borderline forms (BT, BL, LL) | Lepromatous forms (LL, BL) | | **Mechanism** | Th1-mediated, TNF-α driven | Immune complex deposition | | **Antigens involved** | *M. leprae* antigens | *M. leprae* antigens + host antigens | | **Pathology** | Granulomatous inflammation | Neutrophilic inflammation, vasculitis | ### ENL Pathogenesis 1. Occurs in lepromatous leprosy (LL) and borderline lepromatous (BL) forms 2. High bacillary load → abundant mycobacterial antigens 3. Immune complexes form and deposit in tissues (skin, joints, eyes, nerves) 4. Complement activation → neutrophilic inflammation 5. Presents as painful nodules, fever, and systemic symptoms **High-Yield:** ENL is immune complex-mediated; it typically occurs in patients with high bacillary loads and is managed with thalidomide or corticosteroids, not just anti-leprosy drugs. **Mnemonic:** **ENL = Erythema Nodosum Leprosum = Immune complexes** (Type III hypersensitivity in lepromatous leprosy). 
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