IL18 (IFN-gamma-inducing factor, IGIF) is a novel pro-inflammatory cytokine that augments natural killer (NK) activity in spleen cells.
This cytokine can induce the IFN-gamma production of T cells. The combination of this cytokine and IL12 has been shown to inhibit IL4 dependent IgE and IgG1 production, and enhance IgG2a production of B cells. IL18 binding protein (IL18BP) can specifically interact with this cytokine, and thus negatively regulate its biological activity.

IL18 also has other biological properties such as stimulation of Fas ligand expression, and induction of CC and CXC chemokines. IL18 increases production of competent HIV. IL18 is related to the IL1 family. One of the IL18 receptor chains is the IL1 receptor-related protein, a member of the IL 1R family. Like IL1 , IL18 is first synthesized as a leaderless precursor requiring the IL1 converting enzyme (ICE) for cleavage into an active molecule. IL18 may play a role in tissue injury in inflammatory reactions.