IL-6 is a multifunctional cytokine that has an important role in host defense by regulating immune and inflammatory responses. It is expressed by a variety of normal and transformed cells including T cells, B cells, monocytes/macrophages, fibroblasts and various tumor cells. IL-6 has diverse biological functions, it stimulates B-cell differentiation and antibody production, synergizes with IL-3 in megakaryocyte development and platelet production, induces expression of hepatic acute-phase proteins, and regulates bone metabolism. Recombinant human IL-6 is a 20.9 kDa protein containing 184 amino acid residues. Human IL-6 is active on murine cells but murine IL-6 is inactive on human cells.