The extracellular domains of type II IL-1R are structurally related to those of the type I IL-1R; however, the type II IL-1R is a decoy receptor in that the primary ligand, IL-1, preferentially binds to this receptor rather than the signaling receptor. As such, in the presence of increasing expression of the type II receptor on the cell surface, less IL-1 signaling takes place. This is because the type II IL-1R lacks a cytoplasmic domain capable of cell signal transduction. The type II IL-1R also exists in a shed form as a
soluble receptor. The soluble receptor has a high affinity to bind IL-1 over that of IL-1 or IL-1Ra. The soluble IL-1R type II is ideally suited for clinical use because it has a high affinity for IL-1 and a low affinity for IL-1Ra.