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.