CD126 is an 80 kD IL-6 receptor alpha chain also known as IL-6R. It is a member of the immunoglobulin superfamily that is expressed on plasma cells, T cells, activated B cells, monocytes, hepatocytes, epithelial cells, and fibroblasts. Functional IL-6 receptors are formed by the non-covalent association of CD126 and the IL-6 receptor beta chain (CD130 or gp130). CD126 binds IL-6 with low affinity, but does not signal. The beta chain (gp130, CD130) does not bind IL-6 by itself, but associates with the alpha-chain/IL-6 complex to initiate signal transduction. IL-6 binding to the receptor complex results in the stimulation of B and T cells, and hematopoietic precursor proliferation and differentiation. Soluble form of CD126 has been found in human serum.