Interleukin 33 (IL-33), also known as DVS27 or NF-HEV (Nuclear Factor from High Endothelial enules), is a proinflammatory protein and a chromatin-associated cytokine of the IL-1 family with high sequence and structural similarity to IL-1 and IL-18. The encoded protein is involved in the maturation of Th2 cells and the activation of mast cells, basophils, eosinophils and natural killer cells.