Leukapheresis is collection of leukocytes. Leukocytes are also known as white blood cells or immune cells. An automated apheresis machine removes white blood cells from whole blood by density separation and collects them in a collection bag known as a leukopak. The remaining red blood cells and platelets are then returned to the donor. The white blood cells that have been removed will be replaced quickly by your body. Leukapheresis allows for the collection of concentrated, pure white blood cells from a single donor.