IL-18BP is a secreted protein that binds IL-18 with high affinity and prevents its interaction with the IL-18R. Different isoforms of IL-18BP have been demonstrated, four in human and two in the murine system, and biological activity is limited to particular isoforms. IL-18 binding protein (IL-18BP) is a constitutively expressed and secreted protein that resembles the extracellular domains of Ig-like receptors but has no transmembrane form. Because IL-18 is one of the early signals leading to IFN-gamma production by Th1 cells, blocking IL-18 activity by IL-18BP may be involved in down-modulation of the early phases of immune responses.