Interleukin-18 binding protein (IL18BP) is a constitutively secreted protein, with an exceptionally high affinity for IL18 (400 pM) (Kim et al. 2000). It is present in the serum of healthy humans at a 20-fold molar excess compared to IL18 (Novick et al. 2001) and may blunt the Th1 response to foreign organisms, thereby reducing any autoimmune responses to a routine infection (Dinarello et al. 2013). Some studies indicate this IL18BP:IL18 complex negatively regulates IL18:IL18 receptor interaction (Kim et al. 2002, Im et al. 2002).