CD21, also known as IL1RA, is a receptor for interleukin 1. CD21 has been shown to mediate the immunosuppressive signal of interleukin 1, and thus inhibits the synthesis of proinflammatory cytokines. It is structurally related to IFN receptors. It has been reported that CD21 promotes survival of progenitor myeloid cells. Activation of CD21 leads to tyrosine phosphorylation of JAK1 and TYK2 kinases.