The two most important blood group systems are the ABO and the Rh system (Table 7.2).
The ABO System
There are 2 antigens (a substance that stimulate the body to produce antibodies) and 2 antibodies (a protein substance developed in response to, and interacting specifically with an antigen) in the blood, all naturally occurring but not all of them occur in all people. The antigens are found in the red blood cells and are referred to as the A antigen and the B antigen. The antibodies occur in the serum and are referred to as the A and B antibodies.
When a patient's blood is cross-matched, the donor's red blood cells are matched with the recipient's serum. Group O classified as the universal donor, because the red cells of the group O donor had no antigens to react with the antibodies in the recipient's serum. O Rh negative blood is used in an emergency when there may be a delay in obtaining group-specific blood. Group AB was referred to as the universal recipient, because the serum of the group AB patient had no antibodies which would react with the donor's antigen. Unmatched group-specific blood (i.e. blood of the same ABO and Rh group) can be used in an emergency when the patient's blood group is known. It has a very low risk of transfusion reaction and can be used while definitive cross-match is being undertaken.
Direct cross-match: One drop of the recipient's serum and one drop of the donor's blood are placed on a slide and then observed after 10 - 15 minutes. If there is any evidence of clumping, then the two bloods are not compatible.
The Rh System
The Rh antigen is called the D antigen. If this D antigen is present, the patient is said to be Rh positive. If it is absent, the patient is said to be Rh negative. The antibodies to the D antigen do not occur naturally, so if the patient lacks the D antigen it does not always follow that the patient has an (anti D) antibody. The antibody to the D antigen (anti D) occurs only if Rh-positive blood is transfused into an Rh-negative patient. The antigen can also pass from an Rh-positive fetus to Rh-negative mother and cause the production of d antibodies. These antibodies can return to the fetus and affect it adversely. Such effects on the fetus increase with each subsequent pregnancy as the antibody level in the mother builds up. In earlier pregnancies the baby will be jaundiced, a condition referred to as erythroblastosis fetalis.
Rules to administer Rh- blood:
- No woman of child-bearing age should receive Rh-positive transfusion if she is Rh negative.
- No Rh-negative patient at any age should receive Rh-positive transfusion if a previous Rh-positive blood transfusion has been given.
- If available, anti-Rh immunoglobulins should be administered to Rh-negative mothers at delivery if the baby is Rh positive.
Rh-negative blood can be given if uncross-matched blood is required in a hurry because it contains no antibodies. Therefore, if uncross-matched blood is required, group specific Rh-negative blood is safest.