IL4 is a cytokine produced by CD4+ T cells in response to helminthes and other extracellular parasites. It promotes the proliferation and differentiation of antigen presenting cells. IL4 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. IL4 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.