Plasmapheresis is the process in which plasma is separated from the blood cells. After the plasma has been treated, they either put it back, or replace it with saline solution. A situation that someone would need plasmapheresis is if they had an auto-immune disease. Certain auto-immune diseases can be treated with plasmapheresis, such as: myasthenia gravis, Guillain-Barré syndrome, or chronic inflammatory demyelinating polyneuropathy.