IL-10 is an anti-inflammatory cytokine that is produced by T cells, NK cells and macrophages. IL-10 initiates signal transduction by binding to a cell surface receptor complex consisting of IL-10 RI and IL-10 RII. Binding of IL-10 leads to the activation of Jak1 and Tyk2, which phosphorylates Stat-3 . The anti-inflammatory activity of IL-10 is due to its ability to block signaling through other cytokine receptors, notably IFN(gamma) receptor, by upregulating expression of SOCS-1. In addition, IL-10 promotes T cell tolerance by inhibiting tyrosine phosphorylation of CD28. IL-10 is an important negative regulator of the immune response, which allows for maintenance of pregnancy . In contrast, increased IL-10 levels contribute to persistent Leishmania major infections.