IL21 and neoplasm: In instances of tumor diseases, Th9 cells secrete the cytokine IL-9 to exert immune response on other cells, and they additionally secrete interleukin-21 (IL-21) and interleukin-3 (IL-3) to exert divergent impacts on other cells, such as DCs, cytotoxic T lymphocyte cells (CTLs), NKs, mast cells, and others.