Bovine Interleukin-9 (IL-9) is a pleiotropic cytokine primarily produced by activated CD4⁺ T helper cells—particularly Th9 cells—and to a lesser extent by Th2 cells, mast cells, and innate lymphoid cells in cattle (Bos taurus). IL-9 is a member of the common gamma chain (γc) cytokine family, which includes IL-2, IL-4, IL-7, IL-15, and IL-21, all of which signal through receptor complexes that share the γc subunit and activate JAK/STAT pathways. Bovine IL-9 signals through the IL-9 receptor (IL-9R) in combination with the γc chain, primarily activating STAT5 to regulate cell proliferation, survival, and immune modulation. In cattle, IL-9 is associated with type 2 immune responses and plays a role in mucosal immunity, anti-parasitic defense—particularly against gastrointestinal nematodes—mast cell expansion, and regulation of inflammatory responses in respiratory and intestinal tissues. Elevated IL-9 expression has been observed in parasitic infections and allergic-type inflammatory conditions, where it may contribute to both protective immunity and inflammatory pathology. Although less extensively characterized than some other bovine cytokines, IL-9 is increasingly recognized for its role in shaping Th2-biased responses and coordinating immune cell cross-talk. In livestock health and research settings, characterization of bovine IL-9 and related γc family cytokines supports studies of parasite resistance, vaccine responses, allergic inflammation, and immune regulation, contributing to improved disease management, productivity, and welfare in beef and dairy production systems.