apheresis, or hemapheresis, any procedure in which blood is drawn from a donor or patient and a component (platelets, plasma, or white blood cells) is separated out, the remaining blood components being returned to the body. Apheresis allows the donor's blood volume to replenish itself much more quickly than whole blood donation. One type of apheresis, plasmapheresis, is commonly used in commercial blood banks. In plasmapheresis the plasma (the liquid portion of the blood) is separated from donated blood, the red blood cells being returned to the donor. In some diseases, such as myasthenia gravis, plasmapheresis is used to attempt to remove the disease-causing substances from the blood.