Interleukin 19 （IL-19） is an immunosuppressive protein that belongs to the IL-10 cytokine subfamily.The IL-19 protein is composed of 159 amino acids and has a quaternary structure with alpha helix motifs and loops. IL-19 is preferentially expressed in monocytes, macrophages, and T and B lymphocytes, but interacts with immune cells （macrophages, T cells, B cells） and non-immune cells （endothelial cells and brain resident glial cells, etc）. IL-19 is associated with broad functions across inflammation, cell development, viral responses, and lipid metabolism. As an immunosuppressive cytokine, IL-19 promotes the Th2 （regulatory） T-cell response which supports an anti-inflammatory lymphocyte phenotype, dampens the Th1 T-cell response and inflammatory cytokine secretion （IFNγ）, increases IL-10 （anti-inflammatory） expression in peripheral blood mononuclear cells （PBMC）, and inhibits the production of immunoglobulin G （IgG） from B cells.