Blood transfusion centres use cell separators to harvest granulocytes, platelets or plasma from donors. These machines can also be used to replace a patient’s plasma containing harmful components with crystalloid or colloid solutions (plasma exchange, therapeutic plasmapheresis) or to remove an excess of cells (therapeutic cytopheresis). All cytopheresis is done on cell separators which use centrifugal force to separate different blood components according to their relative densities. Plasma exchange can be carried out either by using a centrifugal cell separator or by using membranes to separate plasma from formed elements of whole blood.