Donor blood processing. Bags of donor blood hung from a rail during leucodepletion. Leucodepletion is the removal of white blood cells (leukocytes) from blood samples. Gravity pulls the blood through filters (hanging red squares), which remove most of the white blood cells. White blood cells are removed to lessen adverse transfusion reactions, remove cytomegalovirus and as a precaution against the transmission of vCJD (variant Creutzfeldt Jacob disease), the human form of BSE (bovine spongiform encephalopathy).