A patient on MDT for multibacillary leprosy develops a sudden onset of painful red nodules on the face and extremities with fever and neutrophilia. Histopathology shows neutrophilic infiltrate and immune complex deposition. Which immunological mechanism underlies this reaction?

• A Type 1 reversal reaction — T-cell-mediated shift toward tuberculoid pole
• B Lucio phenomenon — complement-mediated vasculitis
• C Type 2 reaction (ENL) — immune complex-mediated (Type III hypersensitivity) ✓
• D Downgrading reaction — shift toward lepromatous pole

Explanation

Erythema nodosum leprosum (ENL) is a Type 2 leprosy reaction characterized by immune complex deposition (Type III hypersensitivity), with C3 and IgM deposits in vessel walls. It occurs in BL and LL patients, typically precipitated by MDT, pregnancy, or intercurrent illness. Histopathology shows neutrophilic infiltration of existing granulomas and leukocytoclastic vasculitis. Thalidomide is the drug of choice for ENL. Type 1 (reversal) reactions are T-cell-mediated (Type IV) and occur in borderline types.

Reference: Neena Khanna Illustrated Synopsis of Dermatology & STD, 6th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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