leukapheresis

Leukapheresis is a laboratory procedure in which white blood cells are separated from a sample of blood. This may be done to decrease a very high white blood cell count in individuals with cancer (leukemia) or to remove white blood cells for transfusion. In the case of cancer, usually hematological malignancies such as acute leukemias, there are white blood cell counts high enough to cause hemostasis and "sludging" in the capillaries. This can effect retinal vasculature leading to vision changes, pulmonary vasculature leading to shortness of breath from decreased efficiency in oxygen exchange, as well as other organ systems such as the brain which would become clinically apparent with neurological deterioration of a patient from cerebrovascular compromise.
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Leukapheresis /leu·ka·phe·re·sis/ the selective separation and removal of leukocytes from withdrawn blood, the remainder of the blood then being [more]Leukapheresis - Community and Resources

Removal of the blood to collect specific blood cells; the remaining blood is returned to the body.