ILC2‐produced IL‐9 induces mast cells to secret IL‐2, leading to CD25+ ILC‐2 expansion and further, to IL‐9 releasing to prompt ILC2 survival, type 2 cytokines production, Th9 cell activation, and profibrotic TGFβ production by mast cells (Figure 3C).[141] Blocking IL‐9 or inhibiting mast cells may restrict the chronic inflammation and allergy process. The gene discussed is IL9; the disease is allergic disease.