Ovine Interleukin-4 (IL-4) is a Th2-associated cytokine produced primarily by activated CD4⁺ T helper 2 (Th2) cells, mast cells, and basophils in sheep (Ovis aries), where it plays a central role in regulating humoral immunity, alternative macrophage activation, and modulation of inflammatory responses. IL-4 signals through receptor complexes containing IL-4Rα paired with the common gamma chain (γc) or IL-13Rα1, activating JAK/STAT6 pathways that promote B-cell proliferation, antibody class switching, and suppression of Th1-associated cytokines such as IFN-γ. In ovine health, IL-4 is particularly important in immune responses to gastrointestinal nematodes and other parasitic infections, where Th2 polarization supports parasite expulsion, mucosal repair, and development of protective antibody responses. IL-4 also influences immune balance during infections such as ovine progressive pneumonia (maedi-visna), respiratory disease, and mastitis by shaping Th1/Th2 dynamics, which can affect pathogen persistence and inflammatory pathology. While IL-4-mediated responses are protective in promoting humoral immunity and limiting excessive inflammation, excessive or prolonged Th2 skewing may impair effective cell-mediated responses against intracellular pathogens. As a biomarker of type 2 immune activation, ovine IL-4 is relevant in vaccine development, host–parasite interaction studies, and breeding programs aimed at enhancing disease resilience. Characterization of ovine IL-4 supports veterinary research into cytokine network regulation and strategies to improve flock health and productivity through balanced immune modulation.