A procedure in which a machine is used to separate the plasma (the liquid part of the blood) from the blood cells. After the plasma is separated from the blood cells, the blood cells are mixed with a liquid to replace the plasma and are returned to the body. Plasmapheresis is often done to remove extra antibodies, abnormal proteins, or other harmful substances from the blood. It may be used to treat certain types of blood disorders, autoimmune disorders, nervous system disorders, or other conditions. Also called plasma exchange.