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Blood typing involves testing a person’s blood for the presence or absence of certain chemical structures called antigens that are present on the surface of red blood cells. Two of these antigens, are the A and B markers used 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 markers are considered to have blood type O, which is the most common in the Australian population.
Our bodies produce antibodies against those ABO antigens we do not have on our red blood cells, which is why we can only receive blood from donors with 'compatible' blood types.
Another important surface antigen is called RhD or the 'D' antigen. If it is present on your red blood cells, your blood type is Rh-positive; if it is absent, your blood type is Rh-negative.
ABO and RhD blood typing are performed on all donor units collected by the Blood Service and also in the transfusion laboratory for hospital patients. There are two steps to ABO typing: known as 'forward' and 'reverse' typing.
First, forward typing is performed by mixing a sample of blood with a commercial anti-A reagent (that contains antibodies against type A blood) and with anti-B reagent (that contains antibodies against type B blood). Whether the blood cells stick together (agglutinate) in the presence of either of these reagents determines the blood type.
Second, in reverse typing, the patient’s plasma is mixed with blood cells of known A or B types to look for agglutination. A person’s blood type is confirmed by the agreement of these two tests.
Similarly, with RhD typing a sample of a person’s red blood cells is mixed with a commercially-prepared reagent containing D antibodies. If agglutination occurs, then the blood is Rh-positive; if no reaction is observed, then the blood is Rh-negative.
RhD testing is especially important during pregnancy because a mother and her fetus could be incompatible. If the mother is Rh-negative but the father is Rh-positive, the fetus may be positive for the D antigen. As a result, the mother’s body could develop anti-D antibodies which can destroy the baby’s red blood cells.
To prevent development of anti-D antibodies, an Rh-negative mother is treated with an injection of RhD immunoglobulin during the pregnancy and again after delivery if the baby is Rh-positive.
Compatibility testing is performed to determine if a particular unit of blood can be transfused safely into the intended patient. This includes ABO- and RhD blood typing (see above), antibody screening (for unexpected red blood cell antibodies that could cause problems in the recipient), and cross-matching.
There are many antigens besides A, B, and D. However, neither the donor nor the recipient is tested routinely for these other antigens. But, if a patient has had a previous transfusion or been through a pregnancy, 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 plasma with red cells having a known antigenic makeup.
Antibody screening is performed to determine if the patient has antibodies that might react with the donor’s cells. If an antibody is found, the laboratory staff will investigate further to identify the specificity of the 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, anyone can receive type O red blood cells in an emergency. Therefore, people with type O blood (particularly O Rh-negative) are called “universal donors.”
People with type AB Rh-positive blood can be transfused with red blood cells from individuals of any ABO type and are commonly referred to as “universal recipients.”