CD121a is an 80 kD IL-1 receptor type I. It is a transmembrane glycoprotein and a member of the immunoglobulin superfamily expressed on T cells, thymocytes, dendritic cells, fibroblasts, vascular endothelial cells, epithelial cells and neural cells. CD121a binds the mature forms of IL-1alpha, IL-1beta, and IL-1Ra proteins. High affinity binding and signaling by the type I IL-1 receptor requires CD121a and the accessory protein, IL-1R AcP, which does not bind IL-1 alone, but forms a dimer capable of binding IL-1.