Interleukin-4 (IL-4) is a key cytokine produced by activated T cells, mast cells, basophils, neutrophils and eosinophils. IL-4 is critical for the development of Th2-mediated responses, which is related to allergy and asthma. It can also regulate B cell responses, including survival, cell proliferation and gene expression. IL-4 also plays fundamental role for B-cell stimulation, including induction of the IgE isotype switch.