In lepromatous leprosy, despite high bacillary load, the lepromin test is negative. The immunological reason is:

• A Absence of IgM antibodies against M. leprae PGL-1 antigen
• B Complement evasion by M. leprae phenolic glycolipid
• C Deficient Th1 response with predominant Th2 cytokine profile (IL-4, IL-10) suppressing macrophage activation ✓
• D Defective antigen processing due to mutation in TAP1/TAP2 genes

Explanation

Lepromatous leprosy (LL) is characterized by anergy to M. leprae antigens with a Th2-skewed response; IL-4 promotes humoral immunity (high IgG/IgM), and IL-10 suppresses macrophage activation and Th1 differentiation; without effective Th1-driven macrophage activation (IFN-gamma, TNF-alpha), mycobacteria multiply unchecked. Tuberculoid leprosy shows strong Th1 response, positive lepromin test. Absence of IgM anti-PGL-1 would be seen in tuberculoid leprosy (where antibodies are low). TAP mutations cause MHC class I deficiency, not M. leprae-specific anergy.

Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.