Ferret Interleukin-4 (IL-4) is a Th2-associated cytokine produced primarily by activated CD4⁺ T helper 2 (Th2) cells, mast cells, and basophils in ferrets (Mustela putorius furo), 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, and suppression of Th1-associated cytokines such as IFN-γ. In ferret health and research, IL-4 is particularly relevant in respiratory disease models, including influenza virus and SARS-CoV-2 infection, where the balance between Th1 antiviral responses and Th2-mediated inflammation can influence lung pathology, antibody responses, and disease severity. IL-4 may also contribute to airway remodeling and mucus production in allergic or chronic inflammatory conditions. Because ferrets are widely used as translational models for human respiratory viral infections and vaccine studies, characterization of ferret IL-4 supports investigations into Th1/Th2 immune polarization, cytokine regulation, vaccine-induced humoral immunity, and development of targeted anti-inflammatory or immunomodulatory therapies relevant to human allergic and respiratory diseases.