IL-22, also termed IL-TIF, is a member of the IL-10 family of cytokines. Its principal source appears to be memory CD4 T cells with a Th1 polarized phenotype. IL-22 induces its signals through a two-component receptor comprised of IL-22R1 and CRF2-4/IL10Rb. Both of these receptor components also participate in separate receptor complexes specific for other IL-10 family cytokines. Because CRF2-4 exhibits ubiquitous expression, the tropism of IL-22 action appears to be dictated by the expression of IL-22R1. IL-22R1 has a highly restricted expression pattern. Its highest expression, by far, is in the acinar cell population of the pancreas. Lower, but still functional, levels of expression are also observed in skin, colon, liver, and kidney. The responses that have been observed to date for IL-22 resemble the "acute phase" type responses elicited by IL-6, suggesting that IL-22 might be appropriately considered as a T cell-derived IL-6-like activity having distinct target cell specificity. The functional role of this system remains unclear, but it is likely that the responses elicited by this cytokine serve to contribute both to acute host defense against pathogens and to safeguard vulnerable target tissues under conditions of stress.