The bacillus M. leprae is a very small, slightly curved, acid-fast rod. It is an obligate intracellular parasite and it primarily affects the skin and peripheral nerves as it lives within macrophages and Schwann cells. M. leprae grows optimally at 35°C, and therefore has a preference for cooler areas of the body such as the nose, ears, and testicles.
The majority of exposed individuals do not develop disease. Susceptibility and type of response appear to correlate with specific human leukocyte antigen (HLA) types. For example, patients with HLA DR-2 and DR-3 are more likely to develop tuberculoid leprosy, and those with HLA DQ1 usually develop the lepromatous form. Humoral immunity is increased in forms that are associated with low cell-mediated immunity, such as lepromatous leprosy. A Th1 predominance is seen in patients with tuberculoid leprosy, while lepromatous leprosy is associated with a Th2 predominance (16).
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