I have already discussed the function of an erythrocyte.

But now I have come to the royal discipline of transfusion medicine.


Those completely inconspicuous little discs or unfinished doughnuts swim around, squeeze in everywhere to deliver the oxygen and take the CO2 back with them. They don't even have a cell nucleus. As an "average doctor", I greatly respected what these little miracle buggers do. They squeeze into every capillary, even the smallest, to exchange the gases. But my idea was: cell membrane - somewhat strange shape, but necessary for "squeezing through", a cytoskeleton to keep this peculiar shape and then a lot of haemoglobin... that's it. And even under the microscope, the little buggers don't look very impressive. And I only thought a little about blood groups (what they are or where they are). And I'm pretty sure doctors with little to do with blood (ophthalmologists, sports doctors, general practitioners) have a similar idea of erythrocytes. And here comes the great revelation! There are countless molecules on these little discs. They are enzymes, proteins, carbohydrates, transporters or simple molecules of which we still have no idea what they do "for a living". There are currently 376 of these molecules defined. 343 of them are assigned to 43 blood group systems. For 33, the genetics are not yet known. Seven new blood group systems have been added since 2019 alone. And these substances are all blood groups - not just the A and B, but 376. So for me, it was, to put it mildly, a "bit of a surprise" :-).


Of course, the newly discovered blood group antigens do not play a significant role clinically. They are either antigens that we all possess. With these antigens, the absence is the abnormality. Or they can be antigens that are found by chance in someone. These antigens are rare (very rare) and have no significance for the owner.


From a transfusionist's point of view, the first situation is much more critical. This is because if a patient who lacks a high-frequency antigen develops an antibody to that antigen - after a transfusion, pregnancy or other natural stimuli - they are tough to supply with blood. In such patients, the relatives are tested, and if it is possible, such a person should create their own blood depot. Unfortunately, this is rarely possible because only a few blood banks have the equipment to store autologous blood. In Austria, it is only possible in Vienna.


The picture below shows some of the blood groups schematically, but they are not all.


The most important blood group system is the ABO system. It includes four main blood groups, which are defined by two antigens and two antibodies. 

Blood group A is the most common in Europe, with about 43%. The erythrocytes have antigen A on their surface, and the antibodies against blood group B are found in the plasma.

With blood group B, which is one of the rarest with approx. 11%, the B antigen is found on the surface of the erythrocytes, and antibodies against blood group A are in the plasma.

Blood group O is the second most common, with 41%, and the erythrocytes have neither the A nor the B antigen on their surface. In contrast, antibodies against both antigens are found in serum. 

Blood group AB is the rarest (5%). The erythrocytes have both antigens; no antibodies are found in the plasma.


Special features: 


  • Antibodies against the blood group antigens develop during the first 6 months of life. One theory explains the formation of these antibodies by immunisation against intestinal bacteria that have similar structures to the A and B antigens. According to the Landsteiner rule, however, we can only form antibodies against structures we do not have and know, so someone with blood group A does not develop antibodies against bacteria with antigen A-like structures. They only form antibodies against bacteria with antigen B-like structures. With blood group B, it is the other way around.
  • Anti-A and anti-B antibodies belong to the natural (regular) antibodies, also called isoagglutinins. They mainly belong to the IgM antibodies and are more effective at RT or lower. Some of the antibodies in blood group O belong to the IgG antibodies. In addition, some of the antibodies have the specificity anti-AB and cannot be separated into A or B.
  • According to the current state of science, antigens A and B are carbohydrates and have no function.


The presence of these natural antibodies was why the ABO system was discovered first and why it is the most relevant system for us. The first attempts at blood transfusion usually ended fatally. Most likely, the donor did not have a compatible blood group. 


The different properties of serum and red cells were discovered by Karl Landsteiner, who described it in his paper "Ueber Agglutinationserscheinungen normalen menschlichen Blutes". He determined three blood groups: A, B and C, which later became BG O. Landsteiner was awarded the Nobel Prize in Medicine in 1930 for his discovery.


Blood can be transfused either blood group identical or blood group compatible. In this case, it must be ensured that the recipient has no antibodies against the donor erythrocytes. This is called major compatibility in contrast to minor compatibility, which means that the donor has no antibodies against the blood group antigens of the recipient. The minor compatibility is no longer significant with the usual blood products because only a few ml of the donor's plasma remain in a red cell concentrate. Of course, these few isoagglutinins in the donor bag also react with the recipient erythrocytes. However, it happens in a range that is neither measurable nor does it have any clinical relevance.


Only transfusions of whole blood must be transfused identically to the patient's blood group because plasma has a large volume fraction of the transfusion. Whole blood transfusions are no longer common but are used frequently in studies, especially in war zones, where the administration of whole blood is preferred to massive transfusion with individual preparations (erythrocytes, plasma and thrombocytes).


Thus, considering the antigen-antibody combinations listed above, we get the following donor scheme:

An individual with BG O is a universal donor, and an individual with blood group AB is the universal recipient. It is the opposite with plasma - blood group O is the universal recipient, and blood group AB is the universal donor.



This is the donor scheme for erythrocytes and for plasma:

RelevanCE fOr THE CliniCIAN

Before each transfusion of a red blood cell concentrate (blood unit), the transfusing person must perform a bedside test. For a long time, this was a purely medical activity. In August 2016, an amendment to the law extended the administration of blood products to senior nursing staff.


The bedside test is an identity check and should verify the compatibility of the red cell concentrate with the patient for the last time. The test only covers the ABO system. It is done with a quick test to determine the blood group of the red cell concentrate and the patient. To evaluate the test correctly, one has to understand, on the one hand, how it works - and thus, which blood group it indicates. On the other hand, you have to know the compatibility rules. Although we always try to transfuse the blood of the same blood group, if there are irregular antibodies or in the case of massive transfusions, it can happen that no suitable blood unit is found in the patient's blood group. We then have to switch to another compatible blood group. The compatibility rules are shown in the two pictures above. For erythrocyte concentrates, the left scheme applies.


There are 42 other blood group systems - and this number should only be taken with caution - new antigens are found every few months that also acquire the significance of a system. 



The ABO system has been and remains the most important for the clinician. But understanding what Rhesus neg or pos means is not a mistake. Especially gynaecologists should know a bit more about it.


The clinician does not need to know all the other blood group systems. But he should be aware that there is more, and if he ever has a patient who is "difficult to transfuse", he can also understand what it means.

Zuletzt bearbeitet am 13.11.2022.