Interleukin 6 is a multi functional protein that plays important roles in host defense, acute phase reactions, immune responses, and hematopoiesis. IL-6 is expressed by a variety of normal and transformed cells including T cells, B cells, monocytes/macrophages, fibroblasts, hepatocytes, keratinocytes, astrocytes, vascular endothelial cells, and various tumor cells. The production of IL-6 is up-regulated by numerous signals including mitogenic or antigenic stimulation, LPS, calcium ionophore, IL-1, IL-2, IFN, TNF, PDGF, and viruses. IL-6 expression in monocytes is inhibited by IL-4 and IL-13.