Aphe­re­sis is a method for fil­te­ring stem cells out of the blood. This is achie­ved by pas­sing the donor’s blood through a cell sepa­ra­tor. The stem cells are thus extrac­ted by cen­tri­fu­ga­ti­on. The remai­ning blood com­pon­ents are then retur­ned to the donor’s bloo­d­stream. This pro­ce­du­re usual­ly takes 3 – 4 hours.