Immunoglobulin class specific antibodies to M. leprae in leprosy patients, including the indeterminate group and healthy contacts as a step in the development of methods for sero-diagnosis of leprosy.

IgA, IgM and IgG anti-M. leprae antibody activity was quantitated by solid phase radioimmunoassay in groups of untreated leprosy patients throughout the spectrum, in lepromatous leprosy patients treated for more than 10 years, in an indeterminate leprosy group, and in a non-leprosy control group. IgA, IgM and IgG anti-M. leprae antibody activity increased from the group of healthy individuals exposed to M. leprae but without clinical signs of leprosy to tuberculoid (BT and BT/TT) and further to lepromatous (BL to LL) leprosy. There was a considerable overlap in IgA antibody activity, while the overlap between controls and tuberculoid and lepromatous leprosy was less than 20% in the IgM and IgG assays. After more than 10 years of treatment, the IgG anti-M. leprae activity had decreased markedly, whereas there was less effect in the IgA assay and no significant change in the IgM assay. In contrast to earlier findings, the group of 'strictly indeterminate leprosy' showed signs of an active humoral immune response against M. leprae. The IgM anti-M. leprae activity was higher in indeterminate leprosy than in the control group with virtually no overlap. IgA anti-M. leprae was higher in indeterminate leprosy, but with considerable overlap with the controls. No difference between these two groups was found in the IgG assay. The results are discussed in relation to the value of the various immunoglobulin specific anti-M. leprae assays for different purposes, including development of techniques for sero-diagnosis of leprosy.