IL-4 is the primary cytokine implicated in the development of Th2-mediated responses, which is associated with allergy and asthma. The Type I receptor comprises IL-4Rα and the common gamma-chain (γc), which is also shared by the cytokines IL-2, -7, -9, -15 and -21 and is present in hematopoietic cells. IL-4 can use the type II complex, comprising IL-4Rα and IL-13Rα1, which is present in non-hematopoietic cells. This second receptor complex is a functional receptor for IL-13, which shares approximately 25% homology with IL-4. The type I receptor complex can be formed only by IL-4 and is active in Th2 development. In contrast, the type II receptor complex formed by either IL-4 or IL-13 is more active during airway hypersensitivity and mucus secretion and is not found in T cells.