IL-10 is an immunosuppressive cytokine produced by a variety of mammalian cell types including macrophages, monocytes, T cells, B cells and keratinocytes. IL-10 inhibits the expression of proinflammatory cytokines such as IL-1 and TNFα. Like IL-4, IL-10 enhances humoral immune responses and attenuates cell-mediated immune reactions. Human IL-10 active on murine cells, but murine IL-10 is inactive on human cells. Recombinant rat IL-10 is an 18.7 kDa protein of 160 amino acid residues.