Interleukin 9, also known as IL-9, is a pleiotropic cytokine (cell signalling molecule) belonging to the group of interleukins. IL-9 is produced by variety of cells like mast cells, NKT cells, Th2, Th17, Treg, ILC2, and Th9 cells in different amounts. Among them, Th9 cells are regarded as the major CD4+ T cells that produce IL-9.