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 can be found on cells of hematopoietic lineage and is comprised of IL-4Rα and the common gamma-chain (γc). On non-hematopoietic cells, IL-4 can also use the type II complex, comprised of IL-4Rα and IL-13Rα1. 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 binding IL-4 and is active in Th2 development. In contrast, the type II receptor complex is formed by binding either IL-4 or IL-13 is most active during airway hypersensitivity and mucus secretion and is not found in T cells.
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