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IL-18 is a member of the IL-1 family.

IL-18 is a non-glycosylated polypeptide that has a structure similar to IL-1β. Like IL-1β, IL-18 is synthesized as a biologically inactive precursor molecule lacking a classical signal sequence that requires cleavage into an active molecule by IL-1β converting enzyme (ICE, caspase-1).

IL-18 (formerly called interferon gamma inducing factor (IGIF)) is a potent inducer of IFN-gamma production by T cells and NK cells.

Through its induction of IFN-gamma, IL-18 can lead to a rapid activation of the monocyte/macrophage system .

IL-18 induces gene expression and synthesis of tumor necrosis factor (TNF), IL-1, Fas ligand and several cytokines.

In collaboration with IL-12, IL-18 stimulates the Th1-mediated immune responses, but it can also stimulate Th2 immune responses in the absence of IL-12.