Interleukin 1 receptor, type II (IL1R2) also known as CD121b (Cluster of Differentiation 121b) is a cytokine receptor that belongs to the interleukin-1 receptor family. This protein binds interleukin alpha (IL1A), interleukin beta (IL1B), and interleukin 1 receptor, type I (IL1R1/IL1RA), and acts as a decoy receptor that inhibits the activity of its ligands. The pleiotropic cytokine IL1 is produced to regulate the development and maintenance of the inflammatory responses and binds to specific plasma membrane receptors on cells. Two distinct types of IL1 receptors that can bind IL1 specifically have been identified, designated as IL1RI (IL1RA) and IL1RII (IL1RB). IL1R1 contributes to IL-1 signaling, whereas the IL-1R2/CD121b has no signaling property and acts as a decoy for IL-1. IL-1R2/CD121b structurally consisting of a ligand-binding portion comprised of three Ig-like domains, a single transmembrane region, and a short cytoplasmic domain is expressed in a variety of cell types including B lymphocytes, neutrophils, monocytes, large granular leukocytes, and endothelial cells. Interleukin 4 (IL4) is reported to antagonize the activity of interleukin 1 by inducing the expression and release of this cytokine.