IL-10 (Interleukin-10) functions as an anti-inflammatory cytokine by inhibiting the expansion and activation of Th1 cells and Th17 cells and by promoting the development of M2 macrophages and regulatory T cells (Treg). Within a tumor microrenvironment, however, IL-10 can inhibit the expansion of both Treg and myeloid-derived suppressor cells (MDSC). IL-10 exerts protective effects including limiting tissue damage in arthritic inflammation and promoting muscle regeneration after injury, but it also contributes to the persistence of viral infections. IL-10 signals through a receptor complex composed of IL-10 R alpha and IL-10 R beta. IL-10 R beta additionally associates with IL-20 R alpha, IL-22 R alpha 1, or IL-28 R alpha to form the receptor complexes for IL-22, IL-26, IL-28, and IL-29.