Immune response to Mycobacterium leprae in indeterminate leprosy patients.

Immune responsiveness to Mycobacterium leprae was examined in thirtyone histologically classified indeterminate leprosy patients. Fourteen of the patients were also classified as indeterminate clinically (strictly indeterminate group), while the other seventeen patients were clinically classified as tuberculoid or borderline leprosy. The strictly indeterminate group appeared to be quite homogenous in their immune reactivity to M. leprae. All patients revealed a lymphocyte transformation of less than 3 per cent (mean 0.57 + 0.88) and only 1 out of 7 patients tested by the leucocyte migration technique revealed a migration index of less than 0.80 (mean 0.91 + 0.16). Only one patient gave a positive early lepromin reaction. None of the patients revealed a positive reaction in gel precipitation to mycobacterial antigens. These findings are in agreement with the view that the immune response to M. leprae has not been triggered off in strictly indeterminate leprosy. On the other hand, the clinically tuberculoid and borderline patients with an indeterminate histological picture responded on average more strongly to M. leprae, and by and large according to their clinical diagnosis. It is concluded that in classifying patients as indeterminate leprosy, the clinical picture may give more information than histopathology.