Interleukin-4 (IL-4), a member of the α-helical cytokine family, is produced by activated CD4 + T cells, basophils, and mast cells. It promotes the proliferation and differentiation of antigen presenting cells. IL-4 also plays a pivotal role in antibody isotype switching and stimulates the production of IgE. This cytokine has been applied in the treatment of autoimmune disorder like multiple myeloma, cancer, psoriasis, and arthritis. IL-4 has also been extensively applied to inhibit detrimental effect of Th1. It may promote the growth of epithelial tumors by mediating increased proliferation and survival.