IL-6 is produced by many different cell types. The main sources in vivo are stimulated monocytes, fibroblasts, and endothelial cells. Macrophages, T cells and B lymphocytes, granulocytes, smooth muscle cells, eosinophils, chondrocytes, osteoblasts, mast cells, glial cells, and keratinocytes also produce IL-6 after stimulation. IL-6 is a member of a family of cytokines, which also includes LIF, CNTF, Oncostatin M, IL-11, and CT-1. All known members of the IL-6 cytokine family induce hepatic expression of acute phase proteins. The IL-6 receptor is expressed on T cells, mitogen-activated B cells, peripheral monocytes and some macrophage and B cell derived tumor cell types. It is not expressed in resting B cells but in resting T cells. The IL-6 receptor is a strongly glycosylated protein of 80 kDa and a length of 449 amino acids. It has been designated CD126.