Bovine (Bos taurus), Caprine (Capra hircus), and Ovine (Ovis aries) Interleukin-5 (IL-5) is a Th2-associated cytokine produced primarily by activated CD4⁺ T helper 2 (Th2) cells, mast cells, and type 2 innate lymphoid cells in ruminants, where it plays a central role in eosinophil differentiation, activation, recruitment, and survival. IL-5 is a member of the common beta chain (βc) cytokine family, which also includes IL-3 and granulocyte–macrophage colony-stimulating factor (GM-CSF); these cytokines share the βc receptor subunit and regulate hematopoiesis and myeloid cell function. IL-5 signals through a receptor complex composed of IL-5Rα and the shared βc chain, activating JAK/STAT and MAPK pathways that promote eosinophil maturation in the bone marrow and enhance effector responses in peripheral tissues. In cattle, goats, and sheep, IL-5 is particularly important in immune responses to gastrointestinal nematodes and lungworms, where Th2 polarization and eosinophil activation contribute to parasite control and mucosal repair. Elevated IL-5 expression is also associated with allergic-type inflammation and certain respiratory or parasitic conditions, and excessive or prolonged IL-5 signaling may contribute to tissue pathology or fibrosis. As a biomarker of type 2 immune activation, IL-5 is relevant in vaccine development, host–parasite interaction studies, and breeding programs aimed at enhancing parasite resistance in ruminant production systems. Characterization of IL-5 in these species supports veterinary research into Th2 immunity, eosinophil biology, and development of immunomodulatory strategies to improve health, productivity, and disease resilience in livestock.