IL-33 is expressed by several cell types including epithelial cells, endothelial cells, and fibroblasts, and binds a specific receptor, also known as IL1RL1, which is expressed on Tʜ2 cells and mast cells. Signaling via IL1RL1 involves receptor accessory protein (IL1RAP) and leads to the expression of type 2 cytokines, such as IL-4, IL-5, and IL-13. IL-33 is also expressed in blood epithelium and smooth muscle and plays a role in bone protection.
