Human Interleukin 4 (IL-4) is important for immune responses to helminth infection as well as in allergic responses (Olpihant et al.). The IL-4 receptor consists of a heterodimer of IL-4Ra and common gamma chain. IL-4 receptor engagement leads to the activation of JAK1/3 and the recruitment of STAT6 and IRS1/2 (Nelms et al.). IL-4 drives immunoglobulin class switching in B cells (to IgE, IgG4), mast cell hyperplasia, mucus production, and the differentiation of naïve T cells into Th2 cells, which produce IL-4, IL-5, IL-10, and IL-13 (Bao et al.; Nelms et al.; Olpihant et al.). In addition to Th2 cells, IL-4 is produced by CD4+ NK T cells, γ/δ T cells, activated basophils, eosinophils, and mast cells. Human IL-4 does not cross-react with mouse cells (Park et al.).