The process of removing components of a donor's blood and returning the unneeded parts to the donor. The process, also called "hemapheresis," circulates blood from a donor through a filter-type apparatus, and then back to the donor. Apheresis makes it possible to remove desired elements from large volumes of blood. Platelets, red cells, white cells and plasma can be removed separately. For example, this technique permits the harvest of enough platelets for transfusion from one donor (rather than six to eight separate donors). In this way, the recipient of the platelets is exposed to fewer donors or can be given HLA-matched platelets from a single related donor. This technique is also used to remove circulating blood stem cells, which can be frozen and stored for later use in transplant.