IL-9 is a potent, T cell-derived, T cell growth factor which can also enhance mast cell activity and IL-3- or IL-4- dependent proliferation of bone marrow-derived mast cells. IL-9 synergizes with erythropoietin to promote erythroid colony formation. IL-9 has also been reported to protect human T cells from apoptosis induced by IL-2 withdrawal. IL-9 is upregulated in human eosinophils by TNF-alpha and IL1-beta. IL-9 has been reported to downregulate the oxidative burst in activated human alveolar macrophages and induce TGF-beta production. The recently characterized Th-9 helper cell lineage is characterized by the production of IL-9. The MH9A4 antibody reacts with human IL-9. The MH9A4 antibody can neutralize the bioactivity of natural or recombinant IL-9.