A 32-year-old man from rural Odisha presents with hypopigmented patches on his trunk and face with loss of sensation. Slit-skin smear examination shows acid-fast bacilli. Regarding the pathogenesis and immunology of leprosy, all of the following statements are true EXCEPT:

Explanation

## Pathogenesis and Immunology of Mycobacterium leprae ### The Leprosy Spectrum and Immune Response **Key Point:** The immunological response to M. leprae determines the clinical form of leprosy. This is the OPPOSITE of what option 0 claims. **High-Yield:** The Ridley-Jopling classification divides leprosy into a spectrum based on cell-mediated immunity (CMI): | Leprosy Type | CMI Status | IFN-γ Response | Bacillary Load | Lesion Characteristics | |---|---|---|---|---| | **Tuberculoid (TT)** | Strong Th1 | HIGH | Few (paucibacillary) | Well-demarcated, anesthetic | | **Borderline tuberculoid (BT)** | Moderate | Moderate | Moderate | Mixed features | | **Borderline (BB)** | Intermediate | Intermediate | Intermediate | Unstable, variable | | **Borderline lepromatous (BL)** | Weak | LOW | Many | Ill-defined lesions | | **Lepromatous (LL)** | Weak/absent Th1 | LOW | Numerous (multibacillary) | Diffuse infiltration | **Warning:** Option 0 reverses the immunological reality. Lepromatous leprosy is characterized by a **WEAK** Th1 response and **LOW** IFN-γ production, allowing bacilli to proliferate unchecked. Tuberculoid leprosy has the **STRONG** Th1 response. ### Immune Evasion Mechanisms **Mnemonic:** **PGL-1 Mimics Host** — Phenolic glycolipid-1 (PGL-1) is a lipid antigen on M. leprae that: - Mimics host glycoproteins and glycolipids - Suppresses immune recognition - Promotes intracellular survival - Is used diagnostically (anti-PGL-1 antibodies in serology) ### Nerve Involvement Pathophysiology **Clinical Pearl:** M. leprae has unique tropism for peripheral nerve Schwann cells, leading to: 1. Direct invasion and destruction of nerve axons 2. Inflammatory response in the nerve sheath (neuritis) 3. Demyelination and axonal degeneration 4. Permanent sensory and motor deficits if untreated This explains why leprosy is the leading infectious cause of peripheral nerve damage globally. ### Lepromin Test (Mitsuda Test) The lepromin test (intradermal injection of killed M. leprae antigen) measures delayed-type hypersensitivity: - **Positive (>5 mm at 3–4 weeks):** Tuberculoid end (strong CMI) - **Negative:** Lepromatous end (weak CMI) - Correlates with prognosis and treatment response