The role of Th1 cells is well verified in the two main mouse models of L. major infection: the susceptible mouse strain BALB/c shows a weak Th1 and strong Th2 immunity that results from the contribution of distinct factors such as an IL-4-mediated down regulation of the IL-12Rβ on Th2 cells or increased production of IL-12(p40)2 homodimers that antagonize the effect of the IL-12 active form on IL-12R (41, 42, 49); on the other hand, Leishmania-resistant C57BL/6 mice are good controllers of the infection with a strong Th1 response (50–52). This evidence concerns the gene IL4 and infection.