Interleukin 19 (IL-19) is a member of the IL-10 family of cytokines. The IL-10 family is a class II alpha -helical collection of cytokines that contains two groups, a viral homolog and a cellular homolog group. Within the cellular homolog group, there are two additional groupings, one which uses IL-10 R2 as a signal transducing receptor (IL-10, IL-22 and IL-26), and one which uses IL-20 R2 as a signal transducing receptor (IL-19, IL-20 and IL-24). Mouse IL-19 is synthesized as a 176 amino acid (aa) precursor that contains a 24 aa signal sequence and a 152 aa mature region . Based on human studies, it is expected to be secreted as a glycosylated monomer, 35 – 45 kDa in size. IL-19 is unusual in that it contains seven amphipathic helices. Mature mouse IL-19 shares 69% aa sequence identity with the mature human IL-19, and 85% and 68% aa identity to unpublished Genbank sequences for rat and canine IL-19, respectively. Although mouse IL-19 is active on human cells, human IL-19 is not active on mouse cells . IL-19 expression is limited to activated keratinocytes and monocytes, with a possible contribution from B cells . IL-19 binds a receptor complex consisting of the IL-20 receptor alpha (also known as IL-20 R1) and the IL-20 receptor beta (IL-20 R2) . This receptor complex is also shared by IL-20 and IL-24. Notably, IL-19 is reported to actually bind to IL-20 R2, which is generally considered to be only the signal transducing receptor subunit. Functionally, it has been reported that IL-19 both will and will not induce IL-6 and TNF production by monocytes . It does, however, seem to drive T-helper cell differentiation towards a Th2 response, inducing both IL-10 and production of itself .