Interleukin 4 (IL-4) is a cytokine that has pleiotropic effects during active immune responses. Its primary role is to induce differentiation of helper T-cells and active B cells. IL-4’s other roles include regulating immunity, upregulating MHC class II production, supporting the proliferation of epithelial cells, and downregulating the production of Th1 cells, macrophages, and some lines of dendritic cells. IL-4 activates two different receptor complexes, one expressed on hematopoietic cells and another found on nonhematopoietic cells. IL-4 is primarily expressed by Th2-biased CD4+ T cells, mast cells, eosinophils, and basophils. Its responsibilities include cell proliferation, immunoglobulin class switch (IgG4, IgE of human B cells), survival, priming and chemotaxis of mast cells, eosinophils, and basophils. Dysregulation of IL-4 is implicated in many autoimmune disorders, particularly the development of allergies. Overexpression of the cytokine is associated with tumor growth in many kinds of cancer. People infected with HIV often produce higher levels of IL-4. Human, mouse, and rat IL-4 are species-specific in their activities.