IL-33 (Interleukin-33) is released from physically damaged or necrotic cells. It triggers Th2-biased immune cell activation at sites of inflammation as well as regulatory T cell and M2 macrophage expansion. It additionally induces angiogenesis, promotes tumor cell migration and invasion, limits cardiac myocyte hypertrophy, and limits the development of atherosclerotic plaques. IL-33 signals through a receptor complex composed of ST2 and IL-1 RAcP. IL-1 RAcP also associates with IL-1 RI, IL-1 RII, IL-1 R6, and SCF R/c-kit. Soluble isoforms of ST2 and IL-1 RAcP function as decoy receptors that regulate IL-33 bioactivity.