Cynomolgus Monkey (Macaca fascicularis) Interleukin-4 (IL-4) is a Th2-associated cytokine produced primarily by activated CD4⁺ T helper 2 (Th2) cells, mast cells, basophils, and group 2 innate lymphoid cells, where it plays a central role in regulating humoral immunity, allergic inflammation, and alternative macrophage activation. Highly homologous to human IL-4, cynomolgus IL-4 signals through type I (IL-4Rα/γc) and type II (IL-4Rα/IL-13Rα1) receptor complexes, activating JAK/STAT6 pathways that promote B-cell proliferation, IgE class switching, mucus production, and suppression of Th1-associated cytokines such as IFN-γ. In nonhuman primate research, IL-4 is particularly relevant in models of allergic airway disease, asthma, parasitic infection, and vaccine-induced immune responses, where Th1/Th2 balance influences protective immunity and immunopathology. IL-4 also plays a role in shaping immune responses during viral infections such as dengue, influenza, and SARS-CoV-2, where excessive Th2 polarization may modulate antiviral immunity. Because cynomolgus monkeys are widely used in preclinical testing of biologics targeting IL-4/IL-13 pathways and in translational immunology studies, characterization of IL-4 is critical for evaluating immune polarization, safety of cytokine-modulating therapies, and mechanisms underlying allergic and inflammatory disease. As a key regulator of type 2 immunity, cynomolgus IL-4 provides important translational insight bridging nonhuman primate models and human Th2-mediated disorders.