Interleukin 12 (IL-12) is an interleukin that is naturally produced by dendritic cells, macrophages and human B-lymphoblastoid cells (NC-37) in response to antigenic stimulation. IL-12 is composed of a bundle of four alpha helices. It is a heterodimeric cytokine encoded by two separate genes, IL-12A (p35) and IL-12B (p40). IL-12 plays an important role in the activities of natural killer cells and T lymphocytes. It 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. IL12 p40 exists as a monomer and as a homodimer. The IL12B gene contains 8 exons, and maps to 5q31-q33.