IL-12 is composed of a bundle of four alpha helices. It is aheterodimeric cytokine encoded by two separate genes, IL-12A (p35) and IL-12B (p40). The active heterodimer (referred to as ‘p70’), and a homodimer of p40 are formed following protein synthesis. Mature mouse p35 shares 63% and 86% aa identity with human and rat p35, and  mature mouse p40 shares 72% and 93% aa identity with human and rat p40.
IL-12 is involved in the differentiation of naive T cells into Th1 cells. It is known as a T cell-stimulating factor, which can stimulate the growth and function of T cells. IL-12 plays an important role in the activities of natural killer cells and T lymphocytes. IL-12 mediates enhancement of the cytotoxic activity of NK cells and CD8 cytotoxic T lymphocytes. IL-12 also has anti-angiogenic activity, which means it can block the formation of new blood vessels. IL-12 has been shown to both enhance cytotoxic activity and induce interferon-gamma (IFN-γ) production in NK cells, T cells and dendritic epidermal T cells .
Increased plasma levels were found neurological disorders. Significant elevations were measured in autistic patients, and in multiple sclerosis patients. High levels of IL-12 have also been reported for autoimmune diseases and chronic inflammatory reactions, such as in synovial fluid of patients with osteoarthritis, rheumatoid arthritis and seronegative spondylarthropathies, in Sjogren’s syndrome patients and atherosclerosis.