A 32-year-old man with borderline lepromatous leprosy on anti-leprosy therapy for 6 months presents with sudden onset of painful nodules, fever, and neuritis. Skin biopsy shows neutrophilic infiltration with vasculitis. Which single feature best distinguishes this lepra reaction from the other type?

## Lepra Reactions: Type 1 vs Type 2 ### Overview Lepra reactions are immune-mediated complications that occur in borderline forms of leprosy (BT, BL, LL). Understanding the distinction between Type 1 (reversal reaction) and Type 2 (erythema nodosum leprosum, ENL) is high-yield for NEET PG. ### Comparison Table | Feature | Type 1 (Reversal Reaction) | Type 2 (ENL) | |---------|---------------------------|---------------| | **Immunology** | Type IV hypersensitivity (cell-mediated) | Type III hypersensitivity (immune complex) | | **Timing** | Before, during, or shortly after treatment | During treatment or years after completion | | **Leprosy type** | Borderline forms (BT, BL) | Lepromatous/borderline lepromatous (BL, LL) | | **Lesion character** | Inflammation of existing lesions; ulceration possible | Painful nodules, neuritis, iritis, orchitis | | **Histology** | Epithelioid cell infiltration; granuloma formation | Neutrophilic infiltration; vasculitis; immune complex deposition | | **Systemic signs** | Mild fever, localized symptoms | High fever, constitutional symptoms | | **First-line treatment** | Corticosteroids (prednisolone) | Thalidomide (first-line); corticosteroids (adjunct); TNF-α inhibitors | ### Key Point: **Type 2 (ENL) is fundamentally an immune complex–mediated (Type III) reaction**, whereas Type 1 is cell-mediated (Type IV). This is the most reliable discriminator. ### Clinical Pearl: **ENL typically occurs in patients with high bacillary loads** (BL and LL forms), whereas reversal reactions occur in borderline forms with lower loads. ENL can develop years after treatment completion, making it a long-term complication. ### High-Yield: - **Type 1 (Reversal):** Cell-mediated → Corticosteroids - **Type 2 (ENL):** Immune complex → Thalidomide + Corticosteroids ### Mnemonic: **"ENL = Erythema Nodosum Leprosum = Extreme (high bacillary load) + Nodules (painful) + Late (years after Rx)"** ### Mechanism of Type 2 (ENL) 1. High bacillary load → abundant antigen 2. Antigen–antibody complex formation 3. Immune complex deposition in skin, nerves, eyes, testes 4. Complement activation → neutrophilic infiltration and vasculitis 5. Clinical manifestations: painful nodules, neuritis, iritis, orchitis ### Why Thalidomide Works in ENL Thalidomide acts as an immunomodulator: - Inhibits TNF-α production (key cytokine in ENL pathogenesis) - Enhances T-cell proliferation - Reduces neutrophilic infiltration - Response often dramatic (within 24–48 hours) [cite:Park 26e Ch 20]