IL-22 is a cytokine structurally related to IL-10. It was originally identified in mouse as a gene induced by IL-9 in T cells and mast cells. IL-22 was initially designated as ILTIF (IL-10-related T cell-derived inducible factor). IL-22 belongs to a family of cytokines with limited homology to IL-10, namely IL-10, IL-19, IL-20, IL-24, IL-26, IL-28A, IL-28B and IL-29 (the later three also known as IFN-λ). Human IL-22 shares 79% amino acid identity with mouse IL-22 and 25% identity with human IL-10. IL-22 biological activity is initiated by the 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 share sequence similarity with an extracellular region of IL-22R1 (sIL-22R1). Both chains of the IL-22R complex belong to the class II CRF. Two types of IL-22 binding receptors 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 nonimmune cells to regulate local tissue inflammation. As a product of T helper 17 lineage of CD4+ effector lymphocytes, IL-22 plays a critical role in mucosal immunity as well as in deregulated inflammation observed in autoimmune diseases.