Interleukin 13 (IL-13) is a protein that in humans is encoded by the IL13 gene which maps to 5q23-q31. It is cytokine secreted by many cell types, but especially T helper type 2 (Th2) cells, that is a mediator of allergic inflammation and disease. IL-13 is associated primarily with the induction of airway disease, it also has anti-inflammatory properties. IL-13 induces many features of allergic lung disease, including airway hyperresponsiveness, goblet cell metaplasia and mucus hypersecretion, which all contribute to airway obstruction. It also induces secretion of chemokines that are required for recruitment of allergic effector cells to the lung.