The pro-inflammatory cytokine IL-1 induced cellular response requires two subunits of its receptor, IL-1 receptor I (IL-1RI) and IL-1 receptor accessory protein (IL-1RAcP). IL-1RAcP forms a complex with IL-1RI in response to IL-1 treatment. The IL-1 receptor-associated kinase (IRAK), which mediates activation of NF-κB inducing kinae (NIK) and of NF-κB, recruits to the IL-1R complex through IL-1RAcP. IL-1 activation of stress-activated protein kinase and of acid sphingomyelinase also requires IL-1RAcP. Like IL-1RI, IL-1RAcP subunit is essential for IL-1 mediated cellular response. IL-1RAcP is expressed in many tissues.