IL4 and allergic disease: Th9 cells are a major producer of IL‐9 (although other subsets produce small amounts) and mediate immune responses against helminths.[2] In addition, Th9 cells cause allergic reactions[3] and induce antitumor immune responses.[4] The IL‐4 and TGF‐β signaling pathways are important for differentiation of naïve CD4 T cells into Th9 cells.[5] IL‐4 induces expression of STAT6, GATA3, and IRF4,[6] all of which are essential for Th9 cell differentiation.