Apheresis (a-fur-REE-sis) allows volunteers to donate just one blood component. Blood is drawn out through a vein in the arm, and a machine separates the needed component (usually platelets, although red blood cells, white blood cells, and plasma can also be collected this way). The rest of the blood is then returned to the donor, usually through a vein in the other arm. This procedure takes 2 or more hours.
The advantage of this type of donation is that, since most of the blood is returned, a large amount of a needed component can be collected. Patients who need many platelet transfusions, such as some cancer patients, are exposed to fewer donors in this way than they would be from platelets pooled from many donors. This cuts down on the risk of both transfusion reactions and infections.