IL-12 protein is a cytokine produced primarily by monocytes and to a lesser extent by lymphocytes. This cytokine has pleiotropic effects in immunoregulation and inflammation. It down-regulates the expression of Th1 cytokines，MHC class II Ags，and costimulatory molecules on macrophages. It also enhances B cell survival，proliferation，and antibody production. This cytokine can block NF-kappa B activity，and is involved in the regulation of the JAK-STAT signaling pathway. Knockout studies in mice suggested the function of this cytokine as an essential immunoregulator in the intestinal tract.