Th1 activates macrophages and neutrophils to promote an inflammatory response by secreting IL-2, IL-3, IFN-γ, and tumor necrosis factor- (TNF-) α. Th2 can secrete IL-4, IL-5, IL-6, IL-10, and IL-13 to activate mast cells and basophils to participate in allergic reactions. Here, IL4 is linked to allergic disease.