Red Blood Cell (RBC) Antibody Identification

Antibody identification is used as a follow-up test to a positive indirect antiglobulin test (IAT). The IAT is performed as part of a routine ‘group and screen’ or ‘group and crossmatch’ testing panel in a blood transfusion laboratory. The ‘group and screen’ test is part of a routine antenatal test performed during each pregnancy to identify whether the mother may carry any red cell antibodies that may affect the foetus. This test is also performed on any individuals who require, or may require, a blood transfusion in the immediate future. The ‘group and screen’ test procedure is used to determine the ABO and RhD blood group and to screen for the presence of any ‘clinically significant’ antibodies, i.e. any antibodies that would react with donor red cells carrying the target antigen, causing haemolysis in the recipient. The IAT used in the ‘group and screen’ involves mixing the patient serum / plasma (a cell-free fraction of blood) with commercial screening red cells which are known to carry antigens of all the clinically significant blood groups. Thus the fraction of the patient’s blood which contains antibodies is mixed with foreign red cells (screening cells) to see if there is any antigen-antibody reaction. If this test is positive, i.e. there is an antibody in the patient’s blood that reacts with the screening red cells then the red cell antibody identification test must be performed to identify exactly which antibody/antibodies are present.

If one or more clinically significant red cell antibodies are identified in a patient requiring a blood transfusion then compatible antigen-negative donor blood units must be selected for transfusion. This can cause complications for individuals who are transfusion dependent as repeat exposure to donor red cells increases their exposure to foreign antigens, increasing their likelihood of developing antibodies. In turn, the process of finding compatible blood becomes increasingly challenging for such individuals.

An IAT and antibody identification test may also be used as part of an investigation if a person has a transfusion reaction. Very rarely, an RBC antibody may be present in such a small quantity that it does not cause a positive IAT during pre-transfusion blood compatibility testing. But after the blood is given to the recipient, it can trigger renewed, rapid antibody production and cause a delayed haemolytic transfusion reaction several days later.

If an antibody is identified in a pregnant mother, then the father of the baby is tested to predict whether the foetus is likely to carry an antigen that the mother’s antibody may target causing haemolytic disease of the foetus and neonate (HDFN). The most important antibody causing HDFN is anti-D; however, a screening and prophylactic anti-D administration programme is used in this country to reduce the risk of the mother becoming ‘sensitised’ to the RhD positive foetus and producing any antibodies against foetal red cells. If a clinically significant antibody is identified in a pregnant woman then the pregnancy will be closely monitored by her healthcare team who will formulate a care plan based on the nature of the antibody, the presence of the antigen in the foetus and the clinical history. However, the gradual implementation of non-invasive pre-natal testing for RhD genotype will help to reduce the administration of unnecessary prophylactic anti-D in pregnant women.

The antibody identification test may be requested whenever an IAT or a DAT is positive and may be repeated when a person has a transfusion reaction or when a mother has a baby with HDFN.

When a RBC antibody is identified, it means that an antibody that specifically targets an RBC antigen or antigen group is present in the blood. If the antibody is considered clinically significant, then it will need to be taken into account with each transfusion and/or pregnancy. If it is not considered clinically significant, then it is not likely to cause a transfusion reaction in the patient or haemolytic disease of the foetus and neonate.

Examples of red cell antibodies and their clinical significance are shown in the table below. 

Anti-A and anti-B antibodies are naturally occurring, they do not require an initial exposure to the target antigen during blood transfusion, pregnancy, etc. All other blood group antigens must be encountered by an individual’s immune system and recognised as foreign in order for antibodies to be produced.

Transfusion reactions are not exclusively caused by red cell antibodies, the recipient’s immune system can also be triggered when exposed to donor white blood cells and platelets. Very occasionally, antibodies in the plasma of the blood donor may target the red cells of the transfusion recipient. Additionally, a person may develop auto-antibodies that target their own red cell antigens.

Some antibodies may not target a specific red cell antigen but may react with a broad range of red blood cell antigen types, including the patient's own. Such antibodies can occur in association with autoimmune disorders, lymphomas and chronic lymphocytic leukaemia, certain viral or mycoplasma infections, and some medications.

Red cell antibodies can occasionally be missed with antibody identification testing. There are many RBC antigens and some of them are quite rare. Testing evaluates the most common and clinically significant ones. An example of when this might occur is with a person who receives multiple recurrent transfusions and may have a variety of clinically significant and insignificant RBC antibodies. This is why the crossmatching process is important. It evaluates the compatibility of the donor's red blood cells and recipient's serum for each unit of red cells transfused (see Blood Banking).

It is not generally necessary unless someone is pregnant or receiving a transfusion. Red cell antibodies do not otherwise affect the health of someone who has them. Sometimes a doctor may test a woman after a pregnancy, especially if her baby had complications, to determine if there may be risks associated with a future pregnancy. They may also test a person who has received multiple transfusions to evaluate whether red cell antibodies have developed since the last time they were tested.