Also known as plasmapheresis; the removal of the circulating PLASMA. It is done by removing blood from a patient and returning the red cells together with a plasma expander. Usually a sequence of three or four sessions is undertaken, at each of which 2–3 litres of plasma are exchanged.

In autoimmune disorders, damage is caused by circulating ANTIBODIES or sensitised LYMPHOCYTES. If the disease is due to circulating antibodies, their removal from the body should theoretically relieve the disorder. This is the principle on which plasma exchange was based, with such disorders including GOODPASTURE'S SYNDROME, SYSTEMIC LUPUS ERYTHEMATOSUS (SLE) and MYASTHENIA GRAVIS. However, one of the problems encountered is that the body responds to the removal of an antibody from the circulation by enhancing production of that antibody by the immune system. It is therefore necessary to suppress this response with cytotoxic drugs such as AZATHIOPRINE. Nevertheless, remissions can be achieved in autoimmune diseases due to circulating antibodies by the process of plasma exchange.