Apheresis is the process of blood cell separation through the use a machine that draws blood from the circulation, separates out the component of the blood required, and then returns the blood back to the patient. It is used for the collection of peripheral blood stem cells, removal of some cell types or for plasma exchange. It is one of the methods used for harvest of haematopoietic stem cells from donors or patients for transplantation (clickable link to HCT).
Apheresis can also be a therapeutic procedure for the removal of excessive blood cells e.g. for leukocyte reduction in acute leukemia. The process can also be used for the removal of plasma components for plasma exchange in conditions like thrombotic microangiopathy or severe immunological diseases like Guillain-Barre syndrome.