Interleukin-4 (IL-4) is a cytokine, a type of signaling molecule in the immune system. It is primarily produced by T-helper cells, particularly Th2 cells, and plays a crucial role in regulating immune responses. IL-4 promotes the differentiation of B cells into plasma cells, which produce antibodies, and enhances the production of IgE, an antibody associated with allergic reactions. Additionally, IL-4 influences the activity of various immune cells, including macrophages and eosinophils. It helps in the development of T-helper 2 responses, which are important for combating parasitic infections and are also involved in allergic diseases.