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Leprosy is a chronic infectious disease caused by Mycobacterium leprae. M. leprae is an obligate intracellular bacillus which grows best in the cooler areas of the body; skin, peripheral nerves, anterior chamber of the eye, upper respiratory tract, and testes. Its distribution is global and almost exclusively affects individuals in the “developing world”. There is a spectrum of lesions in leprosy; the less severe end of the spectrum is tuberculoid leprosy, and the most severe is lepromatous leprosy.
Tuberculoid leprosy is characterized by the presence of non-necrotizing granulomas in the dermis and peripheral nerves with acid fast bacilli absent or few in number. The lepromin skin test is positive and there is a predominance of helper CD4+ over CD8+ T lymphocytes at sites of infection. Tuberculoid tissues are rich in the mRNAs of the TH1 family of cytokines.
Lepromatous leprosy is characterized by the presence of sheets of foamy macrophages in the dermis (and other sites) containing numerous acid fast bacilli, and absence of granulomas. The lepromin skin test is negative and there is a predominance of suppressor CD8+ over CD4+ T lymphocytes. Lepromatous tissues are rich in the mRNAs of the TH2 cytokines. The loss of ability to kill bacteria appears to be specific for M. leprae. Patients with lepromatous leprosy are not unusually susceptible to opportunistic infections, cancer, or AIDS and maintain delayed-type hypersensitivity to Candida, Trichophyton, mumps, tetanus toxoid, and tuberculin.