The removal of plasma from a patient (usually to treat an immmunologically mediated illness such as thrombotic thrombocytopenic purpura or myasthenia gravis) and its replacement with normal plasma. Plasma exchange therapy can also be used to replace excessively viscous plasma in patients with Waldenstrom’s macroglobulinemia. Pathological (disease-causing) antibodies, immune complexes, and protein-bound toxins are removed from the plasma by plasma exchange.