IL6 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 IL6 after stimulation. IL6 is a member of a family of cytokines, which also includes LIF, CNTF, Oncostatin M, IL11, and CT-1. All known members of the IL6 cytokine family induce hepatic expression of acute phase proteins. The IL6 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 IL6 receptor, designated CD126, is a strongly glycosylated protein of 80 kDa. Recombinant Mouse IL6 is a 22 kDa polypepdtide with two intramolecular disulfide bridges.