Interleukin 33 （IL-33） is a 32kDa proinflammatory cytokine that may also regulate gene transcription in producer cells. 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 12kDa and C-terminal fragment of 18kDa by cleavage of caspase-1.
