IL9 and asthma: Here, Th2 cells start producing large amounts of type‐2 cytokines, which directly or indirectly control all aspects of the type‐2 immune response and induce the hallmark symptoms of asthma (Figure 1A): IL‐5 induces bone marrow eosinophil differentiation and recruitment into the lung; IL‐4 stimulates antibody class switching and IgE production by B cells; IL‐9 promotes the survival and proliferation of histamine‐producing mast cells; IL‐13 reduces the barrier function of epithelial cells and increases mucus production by goblet cells as well as smooth muscle cell hyperreactivity [17, 20].