Pheresis (pronounced as fur-ee-sis) is a special kind of blood donation by which specific components of the blood can be separated. It came from a Greek word meaning 'to take away' or 'separate'. By this method, the blood components like plasma, erythrocytes, platelets, granulocytes (neutrophils and basophils), and agranulocytes (lymphocytes and monocytes) can be separated. The pheresis is done using a machine called a cell separator. The separation is based on density gradient centrifugation The separated components are collected in separate vials or specialized polythene bags in the machine.

The pheresis procedure, or apheresis, is often used to obtain stem cells from the peripheral blood of a person having a blood disease (such as leukemia), which are at a later time reinfused (often after some sort of processing) into that patient's bloodstream in a procedure known as an autologous bone marrow transplant. In this type of transplant, the sick person is his own donor. Similarly, the procedure is often used to obtain stem cells from a healthy person in a procedure known as an allogeneic bone marrow transplant. In this type of transplant, another person (whether related or not) is the donor.

See also Pheresis as treatment for:Myasthenia gravis