A 28-year-old woman with borderline tuberculoid leprosy develops sudden inflammation of existing skin lesions and painful neuritis 2 months after starting anti-leprosy therapy. Biopsy shows epithelioid cell infiltration and granuloma formation. Which feature best distinguishes this reaction from erythema nodosum leprosum (ENL)?

Explanation

## Type 1 Lepra Reaction (Reversal Reaction) vs Type 2 (ENL) ### Clinical Presentation in This Case The patient presents with **inflammation of existing lesions** and **neuritis** during early treatment—classic features of a **Type 1 (reversal) reaction**. The biopsy showing **epithelioid cells and granuloma formation** further confirms Type 1. ### Immunopathological Distinction | Aspect | Type 1 (Reversal) | Type 2 (ENL) | |--------|-------------------|---------------| | **Hypersensitivity type** | Type IV (cell-mediated) | Type III (immune complex) | | **Immune mechanism** | T-cell activation against *M. leprae* antigen | Antigen–antibody complex deposition | | **Histology** | Epithelioid cells, granuloma, lymphocytes | Neutrophilic infiltration, vasculitis, no granuloma | | **Bacillary load** | Low to intermediate (BT, BL) | High (BL, LL) | | **Lesion type** | Inflammation of existing lesions | New painful nodules, systemic involvement | ### Key Point: **Type 1 is a cell-mediated (Type IV) hypersensitivity reaction**, whereas Type 2 is immune complex–mediated (Type III). This is the fundamental and most reliable discriminator between the two. ### High-Yield: - **Type 1 = T-cell response** → Epithelioid granulomas → Corticosteroids - **Type 2 = Immune complex** → Neutrophilic vasculitis → Thalidomide ### Clinical Pearl: **Type 1 occurs in borderline forms (BT, BL)** where the immune system is mounting a delayed hypersensitivity response to *M. leprae* antigens. The inflammation of existing lesions represents a shift in the immune balance toward cell-mediated immunity, often triggered by treatment initiation. ### Mnemonic: **"Type 1 = T-cell (Type IV) = Tuberculoid tendency = Tissue granuloma"** ### Pathophysiology of Type 1 1. Treatment initiates release of *M. leprae* antigens 2. T-cell response strengthens (immune reconstitution) 3. Epithelioid macrophages accumulate in lesions 4. Granuloma formation and inflammation 5. Clinical signs: exacerbation of lesions, neuritis, pain ### Treatment Approach - **First-line:** Prednisolone (0.5–1 mg/kg/day) for 4–12 weeks - **Rationale:** Suppresses T-cell activation and granuloma formation - **Prognosis:** Usually self-limited; resolves with continued anti-leprosy therapy and corticosteroids [cite:Park 26e Ch 20]