Chicken Interleukin-4 (IL-4) is a Th2-associated cytokine produced primarily by CD4⁺ T helper–like cells, basophils, and other type 2 immune populations in chickens (Gallus gallus), where it plays a central role in regulating humoral immunity and modulating inflammatory responses. IL-4 signals through receptor complexes containing IL-4Rα and associated receptor chains, activating JAK/STAT6 pathways that promote B-cell proliferation, antibody production (including IgY and IgA responses), and suppression of Th1-associated cytokines such as IFN-γ. In poultry health, IL-4 is particularly important in responses to parasitic infections such as Eimeria spp. (coccidiosis) and helminths, where Th2 polarization supports mucosal repair and parasite control. IL-4 also influences immune balance during viral infections including avian influenza virus, Newcastle disease virus, and infectious bursal disease virus, shaping the Th1/Th2 response and affecting vaccine-induced immunity. While IL-4-mediated responses are protective in promoting antibody production and limiting excessive inflammation, excessive Th2 skewing may reduce effective cell-mediated immunity against intracellular pathogens. As a biomarker of type 2 immune activation, chicken IL-4 is relevant in vaccine development, disease resistance studies, and genetic selection programs aimed at improving flock health and productivity. Characterization of chicken IL-4 supports research into avian cytokine network regulation and comparative Th1/Th2 immune dynamics across vertebrate species.