Donor blood and recipient blood are typed using the red cell surface ABO and Rh systems and screened for antibodies to other cell antigens. “Direct” cross-matching involves directly mixing the patient’s plasma with the donor’s red cells to establish that hemolysis does not occur from any undetected antibodies. An individual’s red cells display A antigen, B antigen, both antigens (AB), or no surface antigens (O). If the patient’s red cells do not display either surface antigen A or surface antigen B, then antibodies will be produced upon exposure to the absent antigen(s). Thus, a person who is type B may develop anti-A antibodies in the serum, and a type O individual may develop circulating anti-A and anti-B antibodies. A type AB individual will not produce antibodies to either A or B antigens and therefore can receive red cells from any donor blood type. People with type O blood display neither A nor B surface antigens and can donate blood cells to any other type (universal red cell donor; Table 36.2). The universal FFP

donor, conversely, has type AB blood, which will contain neither anti-A nor anti-B antibodies to antigens on recipient RBCs. Whole blood donors and recipients must be exact ABO matches because whole blood contains both RBCs and serum. For example, a unit of type O whole blood will contain serum with anti-A and anti-B antibodies and therefore cannot be used to transfuse patients who are A, B, or AB blood types.

Rh surface antigens are either present (Rh positive) or absent (Rh negative). Individuals who are Rh negative will develop antibodies to the Rh factor when exposed to Rh-positive blood. Although this does not usually have clinical consequences at the time of the initial exposure, subsequent exposures to Rh-positive RBCs may cause circulating antibodies to produce hemolysis. This can pose a particular problem during pregnancy. Maternal anti-Rh antibodies belong to the IgG class and freely cross the placenta. Rh-negative mothers who have developed Rh antibodies will therefore transmit these antibodies to the fetus. If the fetus is Rh positive, massive hemolysis may occur (a phenomenon referred to as hemolytic disease of the newborn or erythroblastosis fetalis). Rh immune globulin (RhoGAM), an Rh-blocking antibody, prevents the Rh-negative patient from developing anti-Rh antibodies. It should be administered to Rh-negative individuals who receive Rh-positive blood and to Rh-negative mothers delivering Rh-positive babies (as some fetomaternal blood mixing often occurs at delivery). The recommended dose is 300 μg intramuscularly for every 15 mL of Rh-positive blood transfused, although administration of Rh-negative blood to Rh-negative mothers is preferred when available.

Recipient antibodies against other donor RBC antigens (such as those from the Kell, Kid, Duffy, or Lewis groups) can also cause hemolytic transfusion reactions. When a patient’s blood sample is screened and found to contain antibodies against antigens found on donor RBCs, it complicates cross-matching and can delay the availability of blood products. When a patient’s antibody screen is positive, it is advisable to discuss anticipated transfusion needs with the blood bank.

If an emergency blood transfusion is needed, type-specific (ABO) red cells can often be obtained within minutes at large centers if the patient’s blood type is known. If type-specific blood is unavailable, type O Rh-negative red cells should be transfused (although type O–positive blood can be used emergently in males and postmenopausal women). Type-specific blood should be substituted as soon as possible to minimize the amount of type O plasma (containing anti-A and anti-B antibodies) transfused.