Four structurally related IL-1 receptor ligands have been described. These include three agonists designated IL-1(alpha), IL-1(beta) and IL-1(gamma)/IL-18 and a specific receptor antagonist, IL-1R(alpha). IL-1(alpha) and IL-1(beta) play critical roles in the regulation of the immune response and inflammation, serving as activators of T and B lymphocytes and NK (natural killer) cells. IL-18 (also referred to as IL-1(gamma)) has been shown to augment the secretion of IFN-(gamma) from T lymphocytes and increase NK cell activity in spleen cells. IL-18 exhibits 19% and 12% identity with IL-1(alpha) and IL-1(beta) respectively over the 12 (beta)-strands of the (beta)-trefoil fold domain, which is a signature feature of the IL-1 family. The unusual leader sequence of IL-18 may be analogous to the IL-1(beta) pro-domain which must be cleaved by the serine protease ICE for optimal secretion and biological activity. Originally described as IGIF (IFN-(gamma)-inducing factor), IL-18 is induced by mouse liver subsequent to challenge with lipopolysaccharide (LPS).