IL-33 (Interleukin-33, NF-HEV) is a nuclear cytokine that is part of the IL-1 family (1,2). Originally discovered as the ‘nuclear factor from high endothelial venules,’ it is widely expressed by endothelial cells as well as epithelial barrier tissues. IL-33 is involved in regulating type 2-associated innate immune responses. It is the ligand for ST2 which, along with IL-1RAcP, forms the IL-33R signaling complex. IL-33 functions as an alarmin upon cellular damage or stress, and is associated with allergic inflammation and asthma.