IL-31, mainly produced by activated CD4+ T cells, is a helical cytokine belonging to the gp130/IL-6 cytokine family. IL-31 is closely related in structure to IL-6, IL-11, and IL-27. Binding of IL-31 to its receptor activates JAK/STAT, PI3K/AKT, and MAPK pathways. IL-31 acts on a broad range of immune and non-immune cells and thus possesses potential pleiotropic physiological functions, such as regulating hematopoiesis and immune response. Transgenic mice overexpressing IL-31 developed severe dermatitis, indicating that IL-31 is involved in skin immunity.