|Like other cytokines, IL-9 is pleiotropic in function. It acts as a growth factor for mast cells and T cells, stimulates mast cell accumulation in tissues, promotes ILC survival, enhances class-switching to IgE in B cells and alters haematopoietic progenitor cell activity . It also has effects on non-haematopoietic cells, such as airway and intestinal epithelial cells, smooth muscle cells and keratinocytes which all express IL-9 receptor α. IL-9 induces chemokine production from these latter cells types, enhances mucus production from airway epithelial cells and alters barrier function in the intestines. The major source of IL-9 is T lymphocytes (e.g. Th9, Th2, Th17 and Treg cells). Many other cell lineages are capable of producing IL-9, including mast cells, innate lymphoid type 2 cells, and NK T cells. The implication of Th9 cells and their sectreted IL-9 in immunopathological disease suggests Th9 cells and/or the IL-9/IL-9 receptor pathway as a target for pharmacological intervention in atopic diseases such as asthma and ulcerative colitis, as well as in cancer and some parasitic infections. The Phase II clinical candidate enokizumab, is an anti-IL-9 monoclonal antibody being evaluated for anti-asthmatic efficacy.