ST2, also known as IL-1 R4, is a component of the receptor for the proinflammatory cytokine Interleukin-33. The IL-33 receptor additionally includes IL-1 RAcp which also associates with IL-1 RI and IL-1 R rp2/IL-1 R6. A soluble form of ST2 functions as a decoy receptor. IL-33 promotes Th2-biased immune responses, resulting in eosinophilia, mast cell activation, and allergic inflammation. In addition, IL-33 enhances inflammation-associated hypernociception and protects from atherosclerosis and cardiac myocyte hypertrophy. Soluble ST2 is elevated in the serum under inflammatory conditions including allergic asthma, sepsis, trauma, dengue fever, pulmonary disease, and lupus. It is also associated aortic stenosis, congestive cardiomyopathy, and risk of cardiovascular heart failure and death.