The IL-18 receptor (IL-18R) complex is composed of two known chains, IL-18 receptor alpha (IL-18Ralpha) and IL-18 receptor beta (IL-18Rbeta). IL-18Ralpha (IL-1R5) is an IL-1 receptor super family member. It was originally described as IL-1 receptor-related protein (IL-1Rrp) because of its homology with the IL-1/Toll receptor family. IL-18Ralpha is the extracellular binding domain of the IL-18 receptor complex. It is expressed on the majority of peripheral CD4+ T cells, Th1 cells, and reciprocally repressed during Th2 development. The expression of IL-18Ralpha is induced by an IL-12/Stat4 pathway and inhibited by IL-4/Stat6 pathway. Binding of IL-18Ralpha together with IL-18beta to proinflammatory cytokine IL-18 plays an important role in innate and autoimmune responses.