ST2, also called IL-1 R4, is an Interleukin-1 receptor family glycoprotein that plays a role in Th2 immune responses. ST2 is expressed on the surface of mast cells, activated Th2 cells, macrophages, and cardiac myocytes. This receptor is very similar to the IL-1 receptor type I and the IL-18 receptor α chain in that ST2 also has three extracellular Ig domains and an intracellular Toll domain. ST2 binds IL-33, enhances inflammatory cytokines by activating nuclear factor-κB (NF-κB) and mitogen activated protein (MAP) kinases. ST2 exists as either a membrane bound form (ST2L) or as a soluble form (sST2). ST2L acts as a transmembrane signalling receptor for IL-33 by mediating the effect of IL-33 on the inflammatory process, while sST2 can suppress IL-33 activity.