Interleukin-18 (IL-18), an 18 kDa cytokine, is a single, non-glycosylated polypeptide chain containing 157 amino acids. IL-18 was previously known as interferon-g inducing factor (IGIF). This pleiotropic cytokine is produced by monocyte/macrophage cells. Like IL-12, IL-18 plays an important role in cell-mediated immune responses.