Rituximab drug molecule. Computer model showing the secondary structure of the drug rituximab (marketed as MabThera). The primary structure of an oligosaccharide (sugar) is at upper left. Oligosaccharides are necessary for the biological activity of proteins. Rituximab is a monoclonal antibody that binds to CD20 receptors on the surface of B cells (white blood cells), flagging them for destruction by the immune system. It is used to treat rheumatoid arthritis, which is caused by the immune system attacking the body's own tissues. By reducing the number of B cells, symptoms can be reduced. It is also used to treat B cell non-Hodgkin's lymphoma and B cell leukaemia as it clears cancerous B cells from the blood, allowing them to be replaced with healthy cells.