Blood typing involves testing a person's blood for the presence or absence of certain antigens that are present on the red blood cells. Two of these antigens, or surface identifiers, are the A and B markers included in ABO typing. People whose red blood cells have A antigens are considered to be blood type A; those with B antigens are type B; those with both A and B antigens are type AB; and those who do not have either of these makers are considered to have blood type O. Our bodies produce antibodies against those ABO antigens we do not have on our red blood cells, which is why we can receive blood only from donors with certain blood types.
Another important surface antigen is the RhD antigen. If it is present on your red blood cells, your blood is D+ (positive); if it is absent, your blood is D- (negative).
According to NHSBT, the distribution of blood types in the UK is as follows:
- O positive: 35%
- A positive: 30%
- O negative: 13%
- A negative: 8%
- B positive: 8%
- B negative: 2%
- AB positive: 2%
- AB negative: 1%
ABO and D blood typing are conducted on all donor units by the collection facility and in the laboratory for hospital patients. There are two steps to ABO typing: forward and reverse typing. First, forward typing is performed by mixing a sample of blood with anti-A serum (serum that contains antibodies against type A blood) and with anti-B serum (serum that contains antibodies against type B blood). Whether the blood cells stick together (agglutinate) in the presence of either of these sera determines the blood type. Second, in reverse typing, the patient's serum is mixed with blood that is known to be either type A or B to watch for agglutination. A person's blood type is confirmed by the agreement of these two tests.
Similarly, with D typing a sample of a person's red blood cells is mixed with an anti-serum containing anti-D antibodies. If agglutination occurs, then the blood is D-positive; if no reaction is observed, then the blood is D-negative. D testing is especially important during pregnancy because a mother and her foetus could be incompatible. If the mother is D-negative but the father is D-positive, the foetus may be positive for the D antigens. As a result, the mother's body could develop antibodies against D, which can destroy the baby's red blood cells. To prevent development of D antibodies, an D-negative mother is treated with an injection of anti-D immunoglobulin during the pregnancy and again after delivery if the baby is D-positive.
Compatibility testing is performed to determine if a particular unit of blood can be transfused safely into a certain patient. This includes ABO and D blood typing (see above), antibody screening (for unexpected red blood cell antibodies that could cause problem in the recipient), and crossmatching.
There are many antigens besides A, B, and D. Some groups of patients, such as those with sickle cell anaemia and thalassaemia, require a more extended match of antigens to prevent antibody formation.
If a patient has had a previous transfusion or has been pregnant, they may have developed antibodies to one of these other antigens. Therefore, it will be important in all future transfusions that the donor's red blood cells do not have that particular antigen; otherwise, the recipient may have a transfusion reaction. The presence of such an antibody is determined by doing an antibody screening test by mixing the patient's serum with red cells of a known antigenic makeup.
Crossmatching is performed to determine if the patient has antibodies that react with the donor's cells. If there is a reaction, the laboratory staff will investigate further to identify the specific antibody and locate donor units that lack the antigen that matches the patient's antibody. This unit will then be tested to confirm that this is a safe match.
It is ideal to receive a blood transfusion with blood that matches your blood type exactly. However, in an emergency, such as major haemorrhage, O D-negative red blood cells may be used. Therefore, people with type O blood (particularly O D-negative) are called "universal donors." People with type AB D-positive blood can be transfused with red blood cells from individuals of any ABO type and are commonly referred to as "universal recipients."