A slit-skin smear from a borderline lepromatous (BL) leprosy patient shows a Bacterial Index (BI) of +4. After 6 months of MDT (multibacillary regimen), the patient develops a new painful nodular erythema of the skin with fever and neuritis. The reaction is classified as ENL (Erythema Nodosum Leprosum). Which immunological mechanism underlies ENL?
- A Type IV delayed-type hypersensitivity mediated by Th1 CD4+ cells releasing IFN-γ
- B Type III hypersensitivity — immune complex deposition with complement activation ✓
- C Type I IgE-mediated mast cell degranulation
- D Type II antibody-dependent cellular cytotoxicity (ADCC) targeting infected Schwann cells
Explanation
ENL (Type 2 lepra reaction) occurs in lepromatous and borderline lepromatous leprosy where there is high bacterial load and abundant IgG antibodies against M. leprae antigens. As MDT rapidly kills bacteria and releases large amounts of antigens, immune complexes form and deposit in tissues, activating complement and triggering Type III (Arthus-like) hypersensitivity. Histology shows neutrophilic infiltration and complement deposition. Treatment is with thalidomide (which inhibits TNF-α) or prednisolone. Type 1 lepra reactions (reversal reactions) in BT-BB-BL leprosy are the Type IV DTH reactions driven by Th1 cells.
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.