Interleukin-4 (IL-4) is a potent lymphoid cell growth factor which stimulates the growth and survivability of certain B cells and T cells. IL-4 is a pleiotropic cytokine that is produced by activated T cells, mast cells and basophils. IL-4 elicits many different biological responses, but has two dominant functions. The first is regulating differentiation of naive CD4+ T cell to the TH2 type. TH2 cells produce IL-4, IL-5, IL-10 and IL-13, which tend to favor a humoral immune response while suppressing a cell-mediated immune response controlled bu TH1 cells. The second is regulating IgE and IgG1 production by B cells.