Interleukin-6 is a multifunctional cytokine that regulates the immune response, haemopoiesis, the acute phase response and inflammation. IL-6 is produced by various types of cell and influences various cell types, and has multiple biological activities through its unique receptor system. IL-6 exerts its biological activities through two molecules: IL-6R (IL-6 receptor) and gp130. When IL-6 binds to membrane-bound form of IL-6R (mIL-6R), homodimerization of gp130 is induced and a high-affinity functional receptor complex of IL-6, IL-6R and gp130 is formed. Interestingly, soluble form of IL-6R (sIL-6R) also binds with IL-6, and the IL-6-sIL-6R complex can then form a complex with gp130. Interleukin-6 receptor is also known as CD126 (Cluster of Differentiation 126).