Equine 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 horses (Equus caballus), 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 their effector activity in peripheral tissues. In equine health, IL-5 is particularly relevant in eosinophil-associated conditions such as equine asthma (recurrent airway obstruction), insect bite hypersensitivity (“sweet itch”), parasitic infections, and certain gastrointestinal inflammatory disorders, where elevated IL-5 contributes to eosinophilic infiltration, airway inflammation, pruritic dermatitis, and tissue remodeling. While IL-5–mediated responses support protective immunity against helminths, excessive or chronic IL-5 signaling can drive persistent allergic inflammation and fibrosis. As a biomarker of type 2 immune activation and eosinophilic disease, equine IL-5 is important in allergy research, vaccine response studies, and immune modulation strategies. In translational research, horses serve as valuable large-animal models for human asthma and allergic airway disease, and characterization of equine IL-5 supports comparative investigations into βc cytokine signaling and development of targeted anti–IL-5 therapies relevant to both veterinary and human medicine.