Helper T cells can be divided into Th1 type cell which produces IL-2 and IFN-gamma and Th2 type cell which produces IL-4, IL-5, and IL-6. In general, cytokine has a pleiotropic action. Furthermore cytokines have redundancy in their action. IL-4 is a major product of Th 2 type cell and is an essential cytokine for IgE production, which is also induced by recently found IL-13. Additionally IL-4 is a key factor for the development of Th2 cell, which produces cytokines responsible for inducing allergic reaction. In this paper, we described the gene regulation, molecular structure and biological action including the anti-tumor activity of IL-4.