ABO blood groups and antibodies
There are four different ABO groups, determined by whether or not an individual’s red cells carry the A antigen, the B antigen, both A and B, or neither. Normal healthy individuals, from early in childhood, make antibodies against A or B antigens that are not present on their own cells.
People who are group A have anti B antibody in their plasma.
People who are group B have anti A antibody.
People who are group O have anti A and anti B antibodies.
People who are group AB have neither of these antibodies.
These naturally occurring antibodies are mainly IgM immunoglobulins. They attack and rapidly destroy red cells.
Red cell units are ABO group compatible if the donor red cells are of the identical ABO group to the recipient. Red cell units with a different ABO group from that of the recipient may also be ABO compatible, as shown below.
Patient’s ABO blood group
Patient’s plasma contains
Red cell units that are compatible
Anti A + B
A B AB O
Thus group O red cell units can be given to a patient of any ABO group in an urgent situation, as the transfused red cells have no A or B antigens to react with the recipient’s antibodies. However, there is a risk of a haemolytic reaction if the patient has antibodies against other red cell antigens. This is most likely if the patient has had pregnancies or has previously been transfused with red cells. In an emergency, the risk of a reaction must be balanced against the risk due to delay in replacing blood loss.
ABO-incompatible red cell transfusion
If red cells of an incompatible ABO group are transfused (and especially if a group O recipient is transfused with group A, B or AB red cells), the recipient’s IgM anti A, anti B and anti AB bind to the transfused red cells. This activates the full complement pathway, causing pores in the red cell membrane and destroying the transfused red cells in the circulation (intravascular haemolysis). The anaphylatoxins C3a and C5a, released by complement activation, will liberate cytokines such as TNF, IL1 and IL8, and stimulate degranulation of mast cells with release of vasoactive mediators. All these substances may lead to inflammation, increased vascular permeability and hypotension, which may in turn cause shock and renal failure. Mediators will also lead to platelet aggregation, lung peribronchial oedema and small muscle contraction. About 20−30% of ABO-incompatible transfusions cause some degree of morbidity, and 5−10% cause or contribute to a patient’s death. The main reason for this relatively low morbidity is the lack of potency of ABO antibodies in group A or B subjects; even if the recipient is group O, those who are very young or very old usually have weaker antibodies that do not lead to the activation of large amounts of complement.
Plasma, cryoprecipitate and platelet concentrates − ABO incompatibility
Transfusion of a small volume of ABO-incompatible plasma is unlikely to cause haemolysis in the recipient. However, infusing a unit of plasma (or cryoprecipitate or platelet concentrate) containing a potent anti A or anti B antibody may haemolyse the recipient’s red cells. Group O plasma and platelet components should only be given to group O recipients.
To learn more, go to www.learnbloodtransfusion.org.uk and study Module 2 in Level 2 − Blood component use.
Diagnosis and management of severe acute transfusion reactions
see Adverse effects.
RhD antigen and antibody
In a Caucasian population, about 15% will lack the RhD antigen and are termed RhD negative. The remainder possess the RhD antigen, and are termed RhD positive. Antibodies to the RhD antigen occur only in individuals who are RhD negative, and follow transfusion or pregnancy. Even small amounts of RhD positive cells entering the circulation of an RhD negative person can stimulate the production of antibodies to RhD. These are usually IgG immunoglobulins. If a woman who is RhD negative develops anti RhD antibody during pregnancy, the antibodies cross the placenta. If the fetus is RhD positive the antibodies destroy the foetal red cells. This will cause haemolytic disease of the newborn (HDN). Without effective management, severe anaemia and hyperbilirubinaemia can develop and may result in severe, permanent neurological damage or the baby’s death (see Obstetric haemorrhage).
Females with potential for childbearing who are RhD negative must not be put at risk of sensitisation by transfusion of RhD positive red cells. If for any reason such a transfusion does occur, administration of anti D immunoglobulin may reduce the risk of sensitisation (see Prevention of HDN).
Other red cell antigen/antibody systems
There are many other antigens on red cells. Transfusion can cause antibodies (alloantibodies) to develop in a recipient if the donor cells express an antigen that the recipient does not posses. Antibodies to red cells are usually detected in patients who have had pregnancies or who have been transfused repeatedly. Some of these antibodies can cause transfusion reactions or damage to the fetus. Before transfusion it is essential to detect potentially harmful antibodies in a patient so that compatible red cells can be selected. It is advisable to avoid transfusion of Kell positive red cells to women of childbearing potential, to avoid risk of HDN due to anti Kell antibodies.
Group and screen
The patient’s blood sample is tested to determine the ABO and RhD type, and to detect red cell antibodies in addition to anti A or anti B that could haemolyse transfused red cells. Provided no such antibodies are present and the patient’s sample is held in the laboratory (usually for up to seven days), the blood bank should generally be able to have compatible blood available for collection in 15−30 minutes without the need for a further sample. Check local procedures.
Red cell compatibility testing (crossmatching)
The patient’s blood is tested to determine the ABO and RhD type, to detect red cell antibodies that could haemolyse transfused red cells, and to confirm compatibility with each of the units of red cells to be transfused.
Electronic issue (computer crossmatch)
Red cell units that are ABO and RhD compatible can be quickly issued for a patient with no further testing, provided there are procedures in place to ensure that:
the patient’s ABO and RhD type have been tested and also confirmed on a second sample, retested on the first sample, or the patient has been found to be group O in the first instance
the patient has no irregular red cell antibodies
the grouping of the blood units is fully reliable
the identification of the patient and his/her sample is fully reliable
the patient’s previous results can be correctly identified and retrieved.
When a second transfusion is required
A fresh sample must be sent to repeat the tests for antibodies if the patient has already had a red cell transfusion more than three days previously, since new antibodies may be stimulated (or low levels of antibodies boosted) as a result of the initial transfusion.
Selecting the correct blood units
The blood bank will use the test results together with the information provided on the request form to select the correct blood component. Special requirements, such as for gamma-irradiated or CMV-negative components, must be indicated on the request form.
Blood ordering for planned procedure
Maximum surgical blood ordering schedule for red cells (MSBOS)
Many operations rarely need transfusion, so there is no need to test, label and reserve blood. This helps to make best use of a restricted stock of red cells in the blood bank. For procedures where transfusion is rarely required, the group and screen or ‘electronic issue’ procedures should be used. For procedures that regularly need transfusion, a surgical team should use a standard blood order that reflects the actual use of blood for their own patients undergoing that particular operation. The MSBOS should be reviewed periodically on the basis of internal audit of blood use.