Interleukin 33 (IL-33) is a cytokine that belongs to the IL-1 superfamily. IL-33 is expressed at a high level in high endothelial venules found in tonsils, Peyer patches and mesenteric lymph nodes. As a type 2 cytokine, IL-33 plays a critical role in helminthic infection and allergic disorders by amplifying immune responses during tissue injury. Studies also suggested that IL-33 might act as an intracellular nuclear factor besides the function of cytokine. Delivery or blockage IL-33 is emerging as a promising therapeutic strategy for maintaining immune homeostasis and protecting against infectious and inflammatory diseases.