There are 5 main Rh antigens on red cells – C, c, D, E, e.
The most important of these is the Rh D. Having the D antigen on the red cell gives you the positive (+) and lacking it gives you the negative (-) after the letter A, B, AB or O.
The D antigen is the most immunogenic, meaning it provokes an immune response that makes it most likely to cause a transfusion reaction in the recipient.
To prevent unwanted transfusion reactions and production of alloantibody, Rh D negative girls and women of child bearing potential should not be transfused with Rh D positive red cells except in an emergency.
Alloantibody produced by transfusion of Rh D positive blood to Rh D negative women, or as a result of specific events during pregnancy, can cause Haemolytic Disease of the Fetus and Newborn (HDFN).
HDFN occurs when, either during a previous transfusion or a previous pregnancy, alloantibody in the mother’s blood crosses the placenta, binding to the fetal red cells. This breaks down the fetal red blood cells causing anaemia, referred to as haemolytic anaemia.
77% of our donors have the D antigen on their red blood cells and are D positive.
The remaining 23% lack the D antigen and are D negative. (Source – NHS Blood and Transplant; Active Donor Base December 2016).
As well as D the Rh blood group system also has CE genes.
CE produces four combinations; ce, Ce, cE or CE
The most frequently occurring phenotypes of Rh CE and Rh D are Dce, cde, DCe, dCe, DcE, dcE, DCE and dCE.
Sometimes we will communicate specifically with groups of donors who have specific phenotypes such as those who have the Dce or the ‘Ro’ subtype or phenotype. While Ro blood is rare among white European people, it is much more common among black African and Afro-Caribbean people.
The Ro phenotype is over 10 times more likely to be found in individuals from a black background than individuals from a white background. It is very frequently found in patients with sickle cell disease.
Ro blood is needed for patients who have the Ro subtype. This ensures the closest possible match for patients and reduces the likelihood of them developing alloantibodies.
Alloimmunisation is exacerbated by differences in blood group distribution between patients with sickle cell disease (SCD) and the predominantly white European blood donor population.
A significant proportion of SCD and thalassaemia patients have the Ro phenotype cDe which is rare in donors of European origin. These groups of patients require ongoing blood transfusions making it important to group and type donors more extensively
(extended phenotype). Transfusion of red cells for these patients should be matched for ABO, D, C, E c, e and Kell.
With demand for Ro donations increasing it is important that we continue our work with the black community to ensure the number of black donors registering and donating increases.
There are many blood groups in the human population including ABO, Rh, Kidd, Kell, Duffy, MNS and Lewis. The most important of these are ABO and RhD. Transfusion with ABO incompatible blood can lead to severe and potentially fatal transfusion reactions. RhD is highly immunogenic and can lead to red cell haemolysis in certain settings.
ABO antigens and antibodies
The ABO blood group is the most important of all the blood group systems. There are four different ABO blood groups (see Table1), determined by whether or not an individual's red cells carry the A antigen, the B antigen, both A and B antigens or neither.
Normal healthy individuals, from early in childhood, make red cell antibodies
against A or B antigens that are not expressed on their own cells. These naturally occurring antibodies are mainly IgM immunoglobulins. They attack and rapidly destroy red cells carrying the corresponding antigen. For example, anti-A attacks red cells of Group A or AB. Anti-B attacks red cells of Group B or AB.
| Name of Blood Group | Antigens present on the red cell surface | ABO antibodies present |
Type O | nil | anti-A and anti-B |
| Type A | A antigen | anti-B |
| Type B | B antigen | anti-A |
| Type AB | A and B antigens | nil |
If ABO incompatible red cells are transfused, red cell haemolysis can occur. For example if group A red cells are infused into a recipient who is group O, the recipient's anti-A antibodies bind to the transfused cells. An ABO incompatible transfusion reaction may result in overwhelming haemostatic and complement activation, resulting in shock, renal failure & death (for more information please click here).
Rhesus D (RhD) antigen
There are more than 40 different kinds of Rh antigens. The most significant Rh antigen is RhD. When RhD is present on the red cell surface, the red cells are called RhD positive. Approximately 80% of the Australian population are RhD positive. The remaining 20% of the population that lack the RhD antigen are called RhD negative.
Antibodies to RhD develop only after an individual is exposed to RhD antigens via transfusion, pregnancy or organ transplantation. Anti RhD (or anti-D) antibodies destroy RhD positive red cells and can lead to haemolytic transfusion reactions. This is of particular importance in pregnancy where anti-D antibodies can cross the placenta from mother to unborn child and lead to haemolytic disease of the newborn.
As a general rule, RhD negative individuals should not be transfused with RhD positive red cells, especially RhD negative girls and women of childbearing age. If transfusion of an RhD positive product to RhD negative recipient is unavoidable a haematologist should be consulted and administration of anti-D immunoglobulin considered.
Are they compatible?
When a transfusion is given, it is preferable for patients to receive blood and plasma of the same ABO and RhD group. However if the required blood type is unavailable, a patient may be given a product of an alternative but compatible group as shown below.
Blood Compatibility | ||
Patient Type | Compatible Red Cell Types | Compatible Plasma Types |
A | A, O | A, AB |
B | B, O | B, AB |
O | O | O, A, B, AB |
AB | AB, A, B, O | AB |
RhD Positive | RhD Positive | RhD Positive |
RhD Negative | RhD Negative | RhD Positive |
Note that Group O RhD negative (O negative) red cells have neither ABO nor RhD antigens on their surface. O RhD negative red cells are issued in emergency situations where life saving transfusion is required prior to completion of a crossmatch. Both RCH and RWH blood banks maintain a reserve of 5 emergency O RhD Negative red cells. (Click here for further information on emergency blood release). Group O is often referred to as the universal red cell donor.
Group AB individuals have neither anti-A nor anti-B antibodies in their plasma. Group AB plasma can therefore be given to patients of any ABO blood group and is often referred to as the universal plasma donor.
Avoiding ABO incompatible transfusions
Most ABO incompatible transfusions occur as a result of improper patient identification at the time of collection of the pre-transfusion sample or administration of the blood product. The pre-transfusion check is carried out at the bedside by 2 members of clinical staff to ensure the right blood is transfused to the right patient. Positive patient identification prior to blood sample collection and labelling the specimen tube at the bedside is critical for accurate sample collection.
Other blood cell antigen-antibody systems
There are many other antigen systems expressed on red cells, white cells and platelets. Transfusion can cause antibodies to develop in the recipient. Some of these antibodies can cause transfusion reactions or damage the foetus. The purpose of pretransfusion testing (or crossmatching) is to detect potentially harmful antibodies in a patient before transfusion and where possible select red cell units that will not react with them.