Interleukin 4 (IL-4) is a cytokine that that functions as a potent regulator of immunity. It is secreted primarily by mast cells, Th2 cells, eosinophils, and basophils. IL-4 signals through the IL-4R α-chain which activates several pathways. This activation can lead to B cell proliferation, survival of T and B cells and chemokine production. IL-4 has been shown to have a role in allergic asthma through its regulation of TH2 cell proliferation and IgE synthesis, and atopy or excessive IgE production has been associated with elevated levels of IL-4 in response to allergens.