Interleukin 33 (IL-33) is a 17.65 kDa cytokine with 159 amino acid residues. IL-33 is a member of the IL-1 family and secreted from various cell types, such as mast cells, macrophages, endothelial cells, and epithelial cells. IL-33 activates NF-κB and MAPK signaling pathways that stimulate the secretion of type 2 cytokines like IL-5 and IL-13 when IL-33 binds to the ST2 receptor primarily expressed in mast cells and Th2 cells. In addition to Th type 2 responses, IL-33 participates in maintaining barrier tissue defense.