While CD4+ T cells are differentiated into Th2 cells in the presence of IL-4 and produce IL-4, IL-5, IL-9, and IL-13 (2, 3), these Th2 cytokines are critical for the development of allergic diseases and the elimination of helminth infections by the induction of IgE synthesis, the activation of basophils and mast cells, and the recruitment of eosinophils. Here, IL4 is linked to allergic disease.