Mycobacterium leprae
It results from the failure of Th1 cell activation which is necessary to eradicate the mycobacteria (Th1 response is required to activate macrophages that engulf and contain the disease). In lepromatous leprosy, TH2 response is turned on, and because of reciprocal inhibition (IL-4; IL-10), the cell-mediated response (TH1) is depressed. Lepromatous leprosy, in contrast to the tuberculoid form of leprosy, is characterized by the absence of epithelioid cells in the lesions. In this form of leprosy "Mycobacterium leprae" are found in lesion in large numbers. This is the most unfavorable clinical variant of leprosy.