Interleukin-1 receptor antagonist (IL-1ra, also known as IL-1F3) is a member of the IL-1 cytokine family that includes IL-1α and IL-1β.IL-1ra inhibits the activity of interleukin-1 by binding to the IL-1R1 receptor and prevents it from binding to IL-1RAP, a co-receptor for signaling. Mouse interleukin-1 receptor antagonist (IL-1ra) is a 22-25 kda glycoprotein produced by a variety of cell types. Cells known to secrete IL-1ra include dermal fibroblasts, vascular smooth muscle cells, intestinal columnar epithelial cells, chondrocytes, macrophages, unkeratinized complex squamous epithelial cells, mast cells, neutrophils and monocytes, support cells, and hepatocytes.There are two types of type I transmembrane glycoprotein receptors for IL-1ra: the biologically active 80 kDa type I IL-1 receptor (IL-1 RI) and an inert 65 kDa type II IL-1 receptor (IL-1 RII).IL-1ra blocks the proinflammatory effects of IL-1, including the release of prostaglandin E2 and IL-6 from endothelial cells, fever, thrombocytosis, hepatic acute-phase protein production, and neutrophil expansion and infiltration.