Plasmapheresis is a medical treatment where blood is removed from a patient's body and put through a centrifuge which separates the red blood cells, white blood cells and plasma. The red blood cells are then returned to the patient, while the plasma is either treated and returned to the patient or discarded and replaced with donor plasma.
It is generally used to treat autoimmune diseases, but can be useful in any disease where the antibodies of the patient are damaged. It can be used to treat Guillain-Barre syndrome, Lupus, thrombotic thrombocytopenic purpura, Myasthenia gravis, granulomatosis with polyangiitis, and Multiple sclerosis.
The treatment works by removing antibodies along with all the other white blood cells. This reduces complications from an overactive immune system.
On the series 
It has been used as a treatment for many patients. It was used to treat Mary in Kids, it was used to treat James in Ignorance is Bliss, It was used to treat Bill in Nobody's Fault, it was used to treat Thad in Risky Business.