IL-6 mediates its biological effects through the type I IL-6 receptor system that consists of two chains, IL-6Rα and gp130. The IL-6Rα chain is the binding component specific to IL-6; while the gp130 only transmits signals of IL-6 when bound to IL-6Rα. The gp130 also can transmit signals from LIF, OSM, CNTF, IL-11 and CT-1 in conjunction with other receptor subunits. The low-affinity binding site for IL-6 is composed of IL-6Rα alone. IL-6Rα is expressed in a wide range of cells including T cells, fibroblasts and macrophages. Soluble IL-6Rα which consists of only the extracellular domain of the IL-6Rα chain, acts as an agonist of IL-6 activity at low concentrations. Recombinant human sIL-6Rα is a 37.6 kDa protein consisting of the extracellular domain of the IL-6Rα chain (338 amino acid residues).