Interleukin 31 (IL-31) is a short-chain member of the alpha helical cytokine family with a molecular weight of 24 kDa that is primarily associated with activated T-cells and is preferentially expressed by Th2 cells over Th1 cells. The human IL-31 cDNA encodes a 164 amino acid precursor consisting of a 23 amino acid signal peptide and a 141 amino acid mature protein. Human and mouse IL-31 share 24% amino acid sequence homology in the mature region. IL-31 signals through a receptor complex consisting of IL-31 receptor A (IL-31RA) and tumor suppressor M receptor subunits. These receptor subunits are expressed in activated monocytes and unstimulated epithelial cells.IL-31 signaling is involved in the Jak/STAT pathway, the PI3 kinase/AKT signaling cascade, and the MAP kinase pathway. In addition, IL-31 plays a role in skin inflammation.