Immunology of leprosy and diagnostic challenges.

Leprosy, caused by noncultivable Mycobacterium leprae (ML), has varied manifestations, which are associated with the host immune responses. The dermal involvement is accompanied by peripheral nerve damage, which leads to sensory motor loss and deformities. Both innate and acquired immune responses are involved. The main cell to be compromised is the CD4 + T helper cell, which shows antigen specific unresponsiveness to only ML and not to other common antigens in the bacilliferous generalized lepromatous form of the disease. In contrast, the paucibacillary localized tuberculoid form shows appropriate T cell functions and poor antibody response. The dichotomy between T cell functions and antibodies are discussed against the current information on cytokines, Th subsets, and regulatory T cells. During lepromatous reactions, there is a temporary, heightened T cell immunity, even in lepromatous subjects. The dermal lesions confirm many features observed with peripheral blood mononuclear cells and give additional information on local immune responses. Nerve damage involves both immune and nonimmune mechanisms. Leprosy is a model disease for understanding host immune responses to intracellular bacilli. There are challenges in diagnosing early leprosy. In spite of intensive efforts by many groups, consensus on a universal test suitable for endemic areas is awaited.