Human Interleukin-4 (IL-4) is a central Th2-associated cytokine produced primarily by activated CD4⁺ T helper 2 (Th2) cells, group 2 innate lymphoid cells (ILC2s), mast cells, and basophils, where it plays a pivotal role in regulating humoral immunity, allergic inflammation, and alternative macrophage activation. IL-4 signals through two receptor complexes: the type I receptor (IL-4Rα paired with the common gamma chain, γc) and the type II receptor (IL-4Rα paired with IL-13Rα1), activating JAK/STAT6 pathways that drive B-cell proliferation, IgE class switching, mucus production, and suppression of Th1-associated cytokines such as IFN-γ. IL-4 is critically involved in allergic diseases including asthma, atopic dermatitis, allergic rhinitis, and food allergy, where it promotes IgE-mediated hypersensitivity, eosinophilic inflammation, and tissue remodeling. While essential for defense against helminths and for coordinating effective antibody responses, dysregulated IL-4 signaling contributes to chronic allergic inflammation and barrier dysfunction. IL-4 also influences tumor immunity and fibrotic processes through macrophage polarization and stromal interactions. As a major therapeutic target, blockade of IL-4Rα (thereby inhibiting IL-4 and IL-13 signaling) has demonstrated clinical efficacy in multiple type 2 inflammatory diseases. Consequently, human IL-4 is central to research on Th1/Th2 immune balance, allergy pathogenesis, immune regulation, and development of targeted biologic therapies.