IL-1 RI, also known as the type 1 IL-1 receptor and CD121a, is a transmembrane protein in the Toll/IL-1 R (TIR) superfamily. IL-1 RI binds the pleiotropic cytokines IL-1 alpha and IL-1 beta, plus the IL-1 receptor antagonist (IL-1 Ra). Signal transduction requires complex formation with IL-1 R AcP/IL-1 R3. This complex recruits the adaptor protein MyD88, to initiate signaling in the NF kappa B pathway. IL-1 RI is expressed predominantly by T cells, fibroblasts, and endothelial cells and mediates acute phase inflammatory responses including fever.