Interleukin-31 (IL-31) is a cytokine with a four-helix bundle structure, which is preferentially produced by type 2 helper T cells (Th2). The structure of IL-31 places it in the IL-6 family of cytokines. IL-31 signals via a receptor complex that is composed of IL-31 receptor A (IL-31RA) and oncostatin M receptor subunits. These receptor subunits are expressed in activated monocytes and in unstimulated epithelial cells. IL-31 is believed to play a role in inflammation of the skin.