Interleukin-6 (IL-6) is a pleiotropic cytokine produced by a variety of cells during infection, trauma, and immunological challenge. IL-6 acts via a receptor complex consisting of two distinct membrane-bound glycoproteins, an 80-kDa IL-6-binding subunit (IL-6R, CD126, gp80) and a 130-kDa signal-transducing element (gp130, CD130). After binding of IL-6 to membrane-bound IL-6R, the complex of IL-6 and IL-6R associates with gp130, thus activating the receptor. Expression of gp130 is found in almost all organs while expression of IL-6R is predominantly confined to hepatocytes and leukocyte subpopulations (monocytes, neutrophils, T cells, and B cells). Soluble form of IL-6R has been found in human serum and urine.