Interleukin 33 (IL-33) is a 32 kDa proinflammatory cytokine and intracellular nuclear factor with transcriptional regulatory properties. IL-33 is structurally related to IL-1, which induces helper T-cells to produce type 2 cytokines and acts through the receptor IL1RL-1 (IL1 receptor-like-1), which is known also as ST2. Binding of IL-33 to this receptor activates NF-kappa-B and MAP kinases and induces in vitro Th2 cells to produce cytokines. In vivo, IL-33 induces expression of IL-4, IL-5, IL-13 and leads to severe pathological changes in mucosal organs and in vitro, it can be divided to N-terminal fragment of 12 kDa and C-terminal fragment of 18 kDa by cleavage of caspase-1.Synonyms: C9orf26, IL-1F11, IL-33, IL1F11, Interleukin-1 family member 11, NF-HEV, NFHEV, Nuclear factor from high endothelial venules