IL-33, a member of the IL-1 family, signals via a heterodimeric receptor complex consisting of ST2 and IL-1R accessory protein and triggers the activation of NF-kB and all three MAPKs (p38, ERK1/2, and JNK1/2) in mast cells. IL-33 is expressed in multiple tissues and by several cell types such as dermal fibroblasts and small airway epithelial and bronchial smooth muscle cells. IL-33 activated IL-1-like signaling responses in mast cells and enhanced IL-5 and IL-13 production from murine Th2-polarized splenocytes. Moreover, it was found that both human and murine mast cells produced a wide spectrum of cytokines and chemokines when stimulated in vitro with IL- 33. In addition, IL-33 enhanced IL-4-driven Th2 cell responses and acted as a selective chemoattractant for Th2 cell recruitment.