IL-13 is the signature cytokine of the type II inflammatory response. It is key player in the inflammatory response triggered either by an invading parasite or allergen. The cellular sources of IL-4 and IL-13 have been studied extensively and along with CD4 T cells, basophils, eosinophils, mast cells, and NK T cells, appropriately stimulated ILC2 cells have the ability to produce IL-4 and IL-13. IL-13 belong to the T helper 2 (Th2) cytokine family, along with IL-3, IL-5, and IL-9. These cytokines are key mediators of allergic inflammation. They have important immunomodulatory activities and exert influence on a wide variety of immune cells, such as B cells, eosinophils, basophils, monocytes, fibroblasts, endothelial cells, airway epithelial cells, smooth muscle cells, and keratinocytes.