Apheresis (ἀφαίρεσις (aphairesis, "a taking away")) is a medical technology in which the blood of a person is passed through an apparatus that separates out one particular constituent and returns the remainder to the circulation. It is thus an extracorporeal therapy. One of the uses of apheresis is for collecting stem cells. Method Depending on the substance that is being removed, different processes are employed in apheresis. If separation by density is required, centrifugation is the most common method. Other methods involve absorption onto beads coated with an absorbent material and filtration. The centrifugation method can be divided into two basic categories: Continuous flow centrifugation Continuous flow centrifugation (CFC) historically required two venipunctures as the "continuous" means the blood is collected, spun, and returned simultaneously. Newer systems can use a single venipuncture by pooling blood in a vessel and cycling through drawing and returning blood though the needle while the centrifuge continuously processes blood remaining in the vessel. The main advantage of this system is the low extracorporeal volume (calculated by volume of the apheresis chamber, the donor's hematocrit, and total blood volume of the donor) used in the procedure, which may be advantageous in the elderly and for children. Intermittent flow centrifugation Intermittent flow centrifugation (IFC) works in cycles, taking blood, spinning/processing it and then giving back the unused parts to the donor in a bolus. The main advantage is a single venipuncture site. It does require a larger extracorporeal volume, and takes significantly longer to perform the procedure via IFC. As such, it is less likely to be used for therapeutic reasons, and is often seen in Donation Center settings. To stop the blood from coagulating, anticoagulant is automatically mixed with the blood as it is pumped from the body into the apheresis machine. Centrifugation variables The centrifugation process itself has four variables that can be controlled to selectively remove desired components. The first is spin speed and bowl diameter, the second is "sit time" in centrifuge, the third is solutes added, and the fourth is not as easily controllable: plasma volume and cellular content of the donor. The end product in most cases is the classic sedimented blood sample with the RBCs at the bottom, the buffy coat of platelets and WBCs (lymphocytes/granulocytes, PMNs, basophils, eosinophils/monocytes) in the middle and the plasma on top. Types There are numerous types of apheresis. Donation Blood taken from a healthy