Red blood cells are also either Rh positive or Rh negative, depending whether you have one of the Rh antigens called D. The D antigen is easily recognized as foreign by an Rh-negative patient whose system then makes an antibody against it.
Blood banks always test red blood cells from donors and transfusion patients to see if they contain A, B and D. They also test the serum (a form of plasma) to see if it contains antibodies to these antigens. The hospital transfusion service also checks the serum of each patient who will be transfused to see if it contains antibodies to other red cell antigens. If it does, the blood bank must provide donor blood lacking these antigens. As a final check, the transfusion service does a cross-match, a test to see if the serum of the patient recognizes anything in the donor blood cells as foreign.
Differences in White Blood Cells The white cells have an entirely different set of foreign antigens than the red cells do. The most important are called HLA antigens. These are the same antigens that a laboratory looks for when tissue typing to select an organ donor for a transplant.
Antibodies to white blood cells develop fairly easily through transfusion or pregnancy. They rarely cause serious harm, but they do explain the chills and fevers experienced by about 3 percent of patients receiving transfusions. If you have such reactions to an uncomfortable degree, your doctor can prevent or minimize them in future transfusions by ordering red blood cells from which most of the white cells have been removed (leukocyte-poor red blood cells) or by transfusing blood through a filter that removes white cells.
