Thalidomide's effectiveness in erythema nodosum leprosum is associated with a decrease in CD4+ cells in the peripheral blood.

Thalidomide is well documented as being an effective drug in the treatment of erythema nodosum leprosum (ENL). The mechanism of action of thalidomide in ENL as well as the pathogenesis of ENL are yet to be fully determined. Lepromatous leprosy patients experiencing ENL have been reported to have an increase in the ratio of CD4+ to CD8+ cells in their blood and ENL skin lesions. Thalidomide has been shown to cause a decrease in the ratio of CD4+ to CD8+ lymphocytes in the blood of healthy males. This decrease was due to a significant reduction in the numbers of Cd4+ lymphocytes and an apparent increase in the numbers of CD8+ lymphocytes. In this study, thalidomide's effectiveness in halting chronic ENL and arresting a relapse into ENL was consistently associated with a decrease in the numbers of CD4+ lymphocytes in the blood of 2 male lepromatous leprosy patients.