Feline Interleukin-4 (IL-4) is a Th2-associated cytokine produced primarily by activated CD4⁺ T helper 2 (Th2) cells, mast cells, and basophils in cats (Felis catus), where it plays a central role in regulating humoral immunity, allergic inflammation, and alternative macrophage activation. 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 production, IgE-associated responses, and suppression of certain pro-inflammatory Th1 cytokines. In feline health, IL-4 is particularly relevant in allergic and immune-mediated conditions such as feline asthma, atopic dermatitis, eosinophilic granuloma complex, and food hypersensitivity, where Th2 polarization drives eosinophilic inflammation and airway or skin pathology. IL-4 may also influence immune balance during chronic viral infections such as feline immunodeficiency virus (FIV) and feline leukemia virus (FeLV) by shaping Th1/Th2 dynamics and potentially affecting viral persistence. As a biomarker of type 2 immune activation, feline IL-4 is important in studies of allergic disease pathogenesis, vaccine responses, and immune regulation. In translational research, cats serve as valuable spontaneous models for human asthma and allergic airway disease, and characterization of feline IL-4 supports comparative investigations into Th2-mediated inflammation and development of targeted anti-cytokine therapies relevant to both veterinary and human medicine.