IL1RL1 and allergic disease: IL-33 is thought to serve as an endogenous danger signal or alarmin in response to tissue damage or inflammation within epithelial and endothelial cells.15,16 ST2 is stably expressed on Th2 cells, mast cells, and group II innate lymphocytes.17 The IL-33/ST2 system has a role in the pathophysiology of a number of inflammatory conditions including allergy,18 atherosclerosis,19 and colitis.20