IL-22, a cytokine structurally related to IL-10, was originally identified in the mouse as a gene induced by IL-9 in T cells and mast cells. IL-22 was initially designated as IL-TIF (IL10-related T cell-derived inducible factor). Mouse IL-TIF consists of 179 amino acids, including four cysteins, and has a predicted molecular weight of 20 kD. Biological activity of IL-22 is initiated by binding to a cell-surface complex composed of IL-22R1 and IL-10R2 receptor chains and further regulated by interactions with a soluble binding protein, IL-22BP, which shares sequence similarity with an extracellular region of IL-22R1 (sIL-22R1). Both chains of the IL-22R complex belong to the class II cytokine receptor families. Two types of IL-22-binding receptor have been discovered, a membrane-bound receptor and a soluble receptor, both encoded by different genes. IL-22 is produced by immune cells and acts on non-immune cells to regulate local tissue inflammation. As a product of the recently identified T helper 17 lineage of CD4(+) effector lymphocytes, IL-22 plays a critical role in mucosal immunity as well as in dysregulated inflammation observed in autoimmune diseases.