Red Blood Cell (RBC) Antibody Screen
Antibody screening tests are performed on whole blood samples that are collected in EDTA anticoagulant. The blood sample is typically obtained by ‘venepuncture’ via a vein in the arm using a needle.
The RBC antibody screen looks for circulating antibodies in the blood directed against red blood cells (RBCs). The primary reason that a person may have RBC antibodies circulating in their blood is because they have been exposed, through blood transfusion or through pregnancy, to RBCs other than their own (foreign RBCs).
Red cells carry many different structures on their cell membrane surface that can act as antigens. An antigen is any substance that may be recognised by the immune system and stimulate an immune response that generates antibodies. The combination of antigens present on the surface of red blood cells determines your blood type. The major red cell antigens include the A, B and Rhesus (Rh) antigens that determine a person’s basic blood types (for more on this, see Blood Type and Blood Banking).
The major blood group antigens on human RBCs are A and B, and a person will be grouped as A, B, AB or O according to the presence or absence of these antigens. Group O individuals do not express A or B antigens on the surface of their RBCs. Another important RBC antigen is the RhD antigen. Individuals who have the RhD antigen are Rh positive (RhD+) and those that do not have the RhD antigen are Rh negative (RhD-) (For more on these antigens, see the article on Blood Typing). The major blood group systems (ABO and Rh) represent only two of the 44 currently recognised blood group systems. These other blood group systems include the Kell, Duffy, Kidd, Lewis, P1PK, MNS and Lutheran groups to name a few.
If there are RBC antibodies present and RBC bearing the corresponding antigens on their surface are introduced into the bloodstream (by transfusion or during pregnancy and delivery) the antibodies rapidly attack and destroy the foreign red cells. This destruction of red cells caused by the anamnestic immune response can result in haemolysis. Antibodies to the ABO antigens are naturally-occurring so do not require exposure to foreign RBCs, most other blood group antigens require an immunising event, or exposure, to stimulate antibody production. An exception to this rule is anti-E, anti-E antibodies can often occur naturally.
There are a few reasons why someone may produce antibodies against RBC antigens.
- Following blood transfusions: Antibodies directed against A and B red cell antigens are naturally occurring; we produce them without having to be exposed to the antigens. These naturally occurring antibodies are present in almost all by the age of 6 months and are very potent. Before receiving a blood transfusion, a person's ABO group and Rh type are matched with that of donor blood to prevent a serious transfusion reaction from occurring. That is, the donor's blood must be compatible with the recipient's so that their antibodies do not react with and destroy donor blood cells.
If someone receives a blood transfusion, their body may also recognise other RBC antigens from other blood groups (such as Kell or Kidd) that they do not have on their own RBCs as foreign. The recipient may produce antibodies to attack these foreign antigens. People who have many transfusions make antibodies to RBCs because they are exposed to foreign RBC antigens with each transfusion.
- With foetal-maternal blood type incompatibility: A baby may inherit antigens from its father that are not present on its mother's RBCs and are therefore recognised as ‘foreign’ by the mother’s immune system. The mother may be exposed during pregnancy or at delivery to the foreign antigens on her baby's RBCs when some of the baby's cells enter the mother's circulation as the placenta separates. The mother may begin to produce antibodies against these foreign RBC antigens. This can cause haemolytic disease of the foetus and neonate, usually not affecting the first baby but affecting subsequent children when the mother's antibodies cross the placenta, attach to the baby's RBCs, and haemolyse them. An IAT can help determine if the mother has produced RBC antibodies other than the expected naturally occurring anti-A and anti-B antibodies.
The first time a person is exposed to a foreign RBC antigen, by transfusion or pregnancy, they may begin to produce antibodies, but their cells do not usually have the time during the first exposure to make enough antibodies to actually destroy the foreign RBCs. This process may be referred to as ‘sensitisation’ or the primary immune response. However, when the next transfusion or pregnancy occurs, the immune system may recognise the foreign antigen and upregulate the immune response (secondary / anamnestic immune response), stimulating production of more antibodies directed against the foreign antigen. This reaction may be strong enough and produce sufficient antibodies to cause haemolysis (destruction) of the transfused RBCs or the baby's RBCs triggering clinical symptoms.
The RBC antibody test screens for the presence of atypical RBC antibodies (other than ABO antibodies (as screening cells are all group O)). If an antibody screen is positive indicating the presence a clinically significant antibody, then antibody identification is required using a commercially available panel of specially selected donor cells.
How is it used?
A RBC antibody screen is used to screen an individual's blood for antibodies directed against red blood cell (RBC) antigens other than the A and B antigens. It is performed as part of a "group and screen" whenever a blood transfusion is anticipated or as part of routine antenatal testing. If an antibody is detected, then an antibody identification test is required to determine which antibodies are present. During a crossmatch, a variation of the RBC antibody screen is performed if clinically significant antibodies are present; this involves mixing the patient serum (a cell-free fraction of blood) with the donor red cells to check if the patient antibody causes agglutination by cross-linking antigens present on donor red cells. In the case of blood transfusions, RBC antibodies must be considered and donor blood must be found that does not contain the antigen(s) to which the person has produced antibodies, in other words, compatible blood.
If someone has an immediate or delayed reaction to a blood transfusion, a direct antiglobulin test (DAT) will be performed to establish if the patients antibodies are coating incompatible donor red cells. A DAT detects antibodies attached to RBCs. If the affected individual has a positive DAT an antibody screen will subsequently be performed to determine if any developed antibodies are incompatible with transfused donor red cells. Antibody identification will be performed to establish if the affected individual has developed any new antibodies.
During pregnancy, the RBC antibody screen is used to screen for antibodies in the blood of the mother that might cross the placenta and attack foetal red cells, causing haemolytic disease of the foetus and new-born (HDFN). The most serious cause is an antibody produced in response to the RBC antigen RhD. A person is considered to be RhD+ if the D antigen is present on their RBCs and RhD- if the D antigen is not present. A RhD- mother may develop an antibody when she is exposed to blood cells from a RhD+ foetus. Therefore, it is important that a RhD- mother has an RBC antibody screen performed early in her pregnancy, typically at booking, at 28 weeks gestation, and again at the time of delivery.
If there are no Rh antibodies present at 28 weeks, then the woman is given an injection of prophylactic anti-D (Rh immunoglobulin (RhIg) to clear any Rh-positive foetal RBCs that may be present in her bloodstream to prevent the production of Rh antibodies by the mother, effectively ‘mopping-up’ foetal red cells before they have a chance to stimulate an immune response.
At birth, the baby's RhD status is determined. If the baby is RhD-, then the mother does not require another prophylactic anti-D (RhIg) injection; if the baby is RhD+ and the mother is RhD-, the mother is given additional prophylaxis (RhIg).
This test may also be used to help diagnose autoimmune-related haemolytic anaemia in conjunction with a DAT. This condition may be caused when a person produces antibodies against their own RBC antigens. This can happen with some autoimmune disorders, such as systemic lupus erythematosus, with diseases such as lymphoma or chronic lymphocytic leukaemia, and with infections such as mycoplasma pneumonia and infectious mononucleosis. It can also occur in some people with the use of certain medications, such as penicillin.
When is it requested?
- A RBC antibody screen is performed prior to any anticipated blood transfusion.
- A RBC antibody screen is performed as part of every woman's antenatal screening assessment. In Rh- women, it is also repeated at 28 weeks, prior to giving a RhIg injection, and after delivery if the baby is found to be Rh+. In Rh- pregnant women with known antibodies, the RBC antibody screen and a dilutional test (known as a titration) may be used as a monitoring tool to track the approximate amount of antibody present throughout the pregnancy.
What does the test result mean?
If a RBC antibody screen is positive, then one or more RBC antibodies are present. Some of these antibodies will be more significant than others, in that they are more likely to cause haemolysis of donor red cells bearing the specific antigen if they are transfused into the body at 37oC. When a RBC antibody screen is performed prior to a blood transfusion, a positive test indicates the need for an antibody identification test to accurately identify the antibodies that are present. Once the antibody has been identified, then donor blood must be found that does not contain the corresponding antigen(s) so that the antibody will not react with and destroy donor RBC antigens following a blood transfusion.
If an Rh- mother has a negative RBC antibody screen, then an Rh immune globulin injection is given within 72 hours to prevent antibody production. If she has a positive test, then the antibody or antibodies present must be identified. If an antibody to the RhD antigen has been actively formed by the mother, then the RhIg injection is not useful. If she has a different antibody, then the RhIg injection can still be given to prevent her from producing antibodies to the RhD antigen.
Is there anything else I should know?
A circulating RBC antibody, once present, will never truly go away but may drop to undetectable levels. Upon re-exposure to the same red cell antigen, antibody is produced rapidly which could induce haemolysis. Therefore, historical RBC antibodies will always be treated as though they are present, even if they are undetectable by laboratory tests. This ensures that an individual with a RBC antibody will not be transfused antigen-positive donor blood.
Each blood transfusion that a person has exposes them to the combination of antigens on the donor RBCs. If transfused RBCs contain antigens foreign to the recipient's RBCs, there is the potential to produce an antibody. If someone has many blood transfusions over a period of time, they may produce antibodies against many different antigens. This can make finding compatible blood increasingly difficult.
What happened before the RhIg (Rh immune globulin) injection was developed?
Prior to development of the injection, RhD- mothers would often become sensitised from the blood of their first RhD+ baby and begin developing anti-RhD antibodies. Any subsequent RhD+ babies would experience some degree of haemolysis due to the mother's anti-RhD antibodies attacking the baby's RBCs. Miscarriages and stillborn babies were relatively common, and those babies who were born often needed immediate blood transfusions to survive. The immunoglobulin injection has largely prevented these complications, although a small percentage of women do still develop RhD antibodies.
I’m blood type O. Do I have a chance of having a baby with ABO haemolytic disease of the foetus and neonate?
Yes. Haemolytic disease of the foetus and neonate may occur when there is an ABO incompatibility between mother and baby, especially with mothers who are blood group O. However, the RBC antibody screen is not useful in this situation because our bodies naturally produce antibodies against the A and B antigens we do not have on our RBCs. A mother who is blood type A will naturally have antibodies directed against the B surface antigens on RBCs, and a mother who is type B will have anti-A antibodies, and so on. Generally, this is a clinically mild haemolytic disease that is easily treatable using phototherapy.
Can I get antibodies from donating blood?