Description: Interleukin-12 (IL-12) is a heterodimeric 70 kD (p70) cytokine composed of two covalently linked, glycosylated chains, 40kD (p40) and 35-kD (p35). IL-12 is mainly produced by monocytes, macrophages, and dendritic cells in response to bacterial products such as lipopolysaccharides (LPS), to intracellular pathogens or upon interaction with activated T cells. IL-12 was originally discovered because of its ability to induce interferon-γ production, cell proliferation, and cytotoxicity mediated by natural killer cells and T cells. It is now established that IL-12 also plays a key role in the development of Th1 responses, leading to IFN-γ and IL-2 production. These cytokines can in turn promote T-cell responses and macrophage activation. The p40 and p35 subunits by themselves have no IL-12 bioactivity, though the p40 homodimer has been shown to bind the IL-12 receptor and to be an antagonist of IL-12 p70. Free p40 is typically secreted in vast excess of IL-12 p70 by cells co-expressing both the p35 and p40 subunits. The p40 subunit of IL-12 has been shown to have extensive amino acid sequence homology to the extracellular domain of the IL-6 receptor, while the p35 subunit shows some homology to IL-6 and G-CSF.