IL-33 (or IL-1F11) was recently identified as a ligand for the previously orphaned IL-1 family receptor T1/ST2. IL-33 belongs to the IL-1 family of cytokines and upon binding to T1/ST2 induces intracellular signals similar to those of IL-1. Previous studies have established that IL-33 and T1/ST2 exert key functions in Th2 responses. Mast cells are known to express T1/ST2. Besides their well-known function in allergy and host defense, mast cells have recently been shown to play an important role in experimental models of arthritis.