IL-1 Receptor Accessory Protein (IL-1 RAcP), also known as IL-1 R3, is a ubiquitously expressed 70-90 kDa member of the Interleukin-1 receptor family of proteins. It serves as a non-ligand-binding component of the receptors for IL-1 alpha, IL-1 beta, IL-33, and IL-36. IL-1 R3 is a coreceptor for IL1RL2 in the IL-36 signaling system. IL-1 R3 is a coreceptor with IL1R1 in the IL-1 signaling system. Associates with IL1R1 bound to IL1B to form the high affinity interleukin-1 receptor complex which mediates interleukin-1-dependent activation of NF-kappa-B and other pathways.