AbstractThalidomide dramatically relieves the signs and symptoms of erythema nodosum leprosum (ENL). ENL is an acute inflammatory complication of lepromatous leprosy. the cause(s) of ENL as well as the mechanism of action of thalidomide in arresting ENL are unknowns.It has been suggested that ENL is the consequence of a transient activation of a cell-mediated-immune (CMI) response toMycobacterium leprae.to initiate a CMI response, an interaction between adhesion and/or signal transducing molecules on T-cells and molecules on antigen presenting cells would occur. An alteration, induced by thalidomide, of one or more of the molecules on T-cells or antigen presenting cells that are essential to maintaining the reactive state of ENL, could explain Thalidomide's ability to attenuate ENL.Thalidomide did not modify: (a) adhesion and/or signal transducing molecules such as CD2, CD4, CD5 and CD8, or (b) molecules that facilitate antigen presentation such as HLA-DR, HLA-A, HLA-B, or HLA-C.