Interleukin 33 (IL-33) is an 18 kDa cytokine with 169 amino acid residues. Interleukin-33 is a member of the IL-1 cytokine family and is primarily expressed in endothelial cells, epithelial cells, fibroblasts, and mesenchymal cells. When IL-33 binds to its receptor, ST2L, it activates mast cells and innate lymphoid cells and subsequently generates IL-5 and IL-13, which play essential roles in allergic inflammation. IL-33 also acts as an alarm by alerting the immune system of cellular damage and plays a critical role in type-2 innate immunity.