Interleukin-6. Molecular model of the secondary structure of the cytokine protein interleukin-6. This protein is produced in the body and has a wide variety of functions in the immune system. These include stimulating the growth and development of B-cells, the white blood cells responsible for the production of antibodies. It also plays a role in inducing nerve cell differentiation and is a growth factor for myelomas and plasmacytomas (white blood cell cancers).