In IL-9-deficient mice, allergic symptoms, such as diarrhea in the food allergy model, are impaired, whereas mice overexpressing IL-9 develop intestinal mastocytosis, intestinal permeability and intravascular leakage.35, 36 These evidences indicated that to suppress allergic reactions in the gut, MC egress, migration/circulation, accumulation and proliferation at local tissues (the latter being stimulated by the cytokines produced by Th2 and Th9 cells, such as IL-3 and IL-9) should be inhibited.36 Here, IL9 is linked to mastocytosis.