In the immunohistochemistry of leprosy lesions, the Ridley-Jopling classification relies on the bacterial index (BI) and morphological index (MI) of AFB in slit-skin smears. Which form of leprosy has the highest bacterial index (6+) and lowest cell-mediated immunity?

• A Tuberculoid leprosy (TT)
• B Lepromatous leprosy (LL) ✓
• C Borderline tuberculoid (BT)
• D Mid-borderline (BB) leprosy

Explanation

Lepromatous leprosy (LL) represents the immunologically anergic pole of the spectrum — very high bacillary load (BI 4–6+), massive infiltration of foamy macrophages (Virchow cells) stuffed with M. leprae, absent granuloma, and undetectable/absent cell-mediated immunity (CMI) as evidenced by negative lepromin (Mitsuda) test. The paradox is that humoral immunity (IgG, IgM antibodies) is intact (positive anti-PGL-1 antibody), but this does not protect against the infection. In contrast, TT has BI of 0 (paucibacillary), strong CMI, positive lepromin test, and well-formed granulomas. LL is classified as multibacillary (BI ≥2+) and treated with WHO-MDT regimen for 12 months.

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

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