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The pleiotropic cytokine IL1 is produced to regulate development and maintenance of the inflammatory responses, and binds to specific plasma membrane receptors on cells. Two distinct types of IL1 receptors which are able to bind IL1 specifically have been identified, designated as IL1RI (IL1RA) and IL1RII (IL1RB). A closely related receptor-like chain IL1R accessory protein (IL1RAcP) has been reported to heterodimerize with the IL1RI to form the signal transduction complex which appears to mediate all the known IL1 signal processes. IL1RII 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. Like the IL1RI, the human IL1RII can bind all three forms of IL1 (IL1 alpha, IL1 beta and IL1ra) but does not transmit IL1 signals. In addition to the membrane-bound form, IL1RII also exists as the soluble form, and either acts as a decoy receptor that inhibits IL1 action by blocking the binding of IL1 to the type I receptor complex. Furthermore, IL4 is reported to antagonize the activity of IL1 by inducing the expression and release of this cytokine. Recombinant soluble IL1RII is thus a potent antagonist of IL1 action, and serves as a potential therapeutic targe.