IL33 is a cytokine which belongs to the IL-1 superfamily, and it induces helper T cells to produce type 2 cytokines. This cytokine was previously named NF-HEV 'nuclear factor (NF) in high endothelial venules' (HEVs), as it was originally identified in these specialized cells.

IL33 mediates its biological effects by interacting with the receptors ST2 and IL-1 Receptor Accessory Protein, activating intracellular molecules in the NF-kappaB and MAP kinase signaling pathways that drive production of type 2 cytokines (e.g. IL-4, IL-5 and IL-13) from polarized Th2 cells. The induction of type 2 cytokines by IL-33 in vivo is believed to induce the the severe pathological changes observed in mucosal organs following administration of IL33.