Interleukin 31 (IL-31) is an 18 kDa cytokine with 142 amino acid residues. IL-31 is mainly secreted from activated CD4+ T cells and binds to a receptor called IL-31RA. Upon binding to 31RA, IL-31 activates several signal pathways like MAPK, PI3K/AKT, and Jak/STAT pathways. It is critical in regulating many biological functions, such as cell proliferation, hematopoiesis, induction of cytokines, inflammation, and immune response.