Rat Interleukin-4 (IL-4) is a Th2-associated cytokine produced primarily by activated CD4⁺ T helper 2 (Th2) cells, mast cells, and basophils in rats (Rattus norvegicus), 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 class switching, and suppression of Th1-associated cytokines such as IFN-γ. In rat models, IL-4 is widely studied in experimental systems of asthma, allergic airway inflammation, atopic dermatitis, parasitic infection, and autoimmune disease, where Th2 polarization drives eosinophilic infiltration, mucus hypersecretion, and tissue remodeling. IL-4 also influences macrophage polarization toward an M2 phenotype, contributing to wound healing and fibrotic responses in models of lung, liver, and cardiac fibrosis. While protective in supporting antibody-mediated immunity and parasite clearance, excessive or sustained IL-4 signaling can impair effective cell-mediated responses and promote chronic allergic pathology. As both a biomarker and mechanistic driver of type 2 immunity, rat IL-4 remains central to preclinical research on Th1/Th2 immune balance, cytokine regulation, and development of targeted therapies for allergic, fibrotic, and inflammatory diseases relevant to human health.