Human Interleukin-4 (IL-4) is a monomeric cytokine, approximately 13-18 kDa, expressed by Th2-biased CD4+ T cells, mast cells, basophils, and eosinophils. IL-4 has a compact, globular fold (similar to other cytokines) stabilized by 3 disulphide bonds. Mature human IL-4 shares 55%, 39%, and 43% amino acid sequence identity with bovine, mouse, and rat IL-4, respectively. By binding to IL-4 receptor or receptor complex, IL-4 has many biological functions. It promotes cell proliferation, survival, and immunoglobulin class switch to IgG4 and IgE in human B cells, acquisition of the Th2 phenotype by naïve CD4+ T cells, priming and chemotaxis of mast cells, eosinophils, and basophils, and the proliferation and activation of epithelial cells. IL-4 plays an important role in the development of allergic inflammation and asthma.