Interleukin-4 (IL-4), also known as BSF-1 and Lymphocyte stimulatory factor 1, is a secreted cytokine belonging to the IL-4/IL-13 family. It is produced by mast cells, T cells and bone marrow stromal cells. IL-4 signals through two receptor complexes: theIL4Ra/common ? chain and the IL4Ra-IL-13 Ra1 receptor complex. IL-4 regulates T cell and B cell proliferation, survival and gene expression. IL-4 also induces IgG and IgE class switching, and the upregulation of MHC-II production.