The evolution of the lesion in cutaneous leishmaniasis.

In order to elucidate the mechanisms for the elimination of Leishmania, the histological evolution of the lesions of cutaneous leishmaniasis was studied in 118 biopsies from four geographical areas known to be associated with different species or sub-species of Leishmania. Basically there were 3 types of response: A, parasites were eliminated within intact macrophages which later evolved as epithelioid cells; B, they were eliminated as a result of the lysis of the macrophages either individually or in small clusters, but the process was incomplete; C, there was necrosis proceeding to completion at the centre of a focalised mass of macrophages. In B and C the release of parasites caused tissue destruction; epithelioid cells were immature and often sparse, though giant cells were seen in C. A more definite tuberculoid response was found in draining lymph nodes. The response (A, B or C) depended partly on the parasite load, partly on geographical factors. The relative proportions of macrophages, plasma cells and lymphocytes in the lesions varied with the parasite index, but the relationship was the same in all 3 types of response. This suggested that the 3 responses might be the outcome of a common immunological mechanism operating at different antigen levels or antigen-antibody ratios.