Blood types

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The differentiation between different blood groups is based on the different surface properties of the red blood cells. There are different blood type systems, the most important of which are the AB0 system and the Rhesus system. Read more about the topic here: How are blood groups divided? For example, what does “blood group 0 Rh positive” mean? Which blood group is rare and which is common? What role does blood group compatibility play in blood transfusions?

What are blood types?

The surface of red blood cells (erythrocytes) consists of various structures such as proteins and lipid compounds. They are called blood group antigens. Everyone has a certain type of such antigens and therefore a certain blood group. The most important blood group systems are the AB0 and Rhesus systems. There are also other blood group systems that can be important in special cases, for example:

• Kell (important in patients who need frequent blood transfusions)
• Duffy
• MNSs
• Kidd
• Lewis

Blood group antibodies

Not all blood groups “get along” with one another. This is due to the blood group antibodies: for each blood group antigen there is a specific blood group antibody that recognizes the antigen in question and can clump with it (or the erythrocyte in question). Blood group antibodies that are directed against the antigen that this person does not have swim in a person's blood. This prevents the immune system from attacking its own erythrocytes. However, if a patient accidentally receives blood from a different blood group during a blood transfusion, the antibodies of which are circulating in his blood system, these fight the foreign erythrocytes.

How many blood groups are there in the AB0 system?

The AB0 system was first described in 1901. It differentiates between four blood groups: A, B, AB and 0. Which blood group a person has depends on the composition of two predispositions (genotypes).

blood type	genotype	Blood group: antibodies
Blood group A	AA or A0	Anti-B
Blood group B	BB or B0	Anti-A
Blood group AB	AWAY	No
Blood group 0	00	Anti-A and Anti-B

How many blood groups are there in the rhesus system?

There are five antigens in the rhesus blood group system: D, C, c, E, and e. The main characteristic is the Rhesus factor D (Rh factor). If a person has this factor on their erythrocytes, they are Rh-positive.If the factor is absent, it is Rh negative.

Further information: Rh factor

What is the rarest blood group, what is the most common blood group?

Blood group AB is particularly rare. It occurs in only about five percent of the population in Germany. Overall, the blood group frequency in Germany looks as follows:

AB0 and Rh blood groups (Germany)	Blood group frequency
Blood group A positive	37%
Blood group A negative	6%
Blood group B positive	9%
Blood group B negative	2%
Blood group 0 positive	35%
Blood group 0 negative	6%
Blood group AB positive	4%
Blood group AB negative	1%

When do you determine the blood groups?

The blood group is determined in the following cases:

• Prevention during pregnancy and for newborns
• Creation of an emergency ID card
• Preparing for a blood transfusion, for example before an operation or if you have severe anemia (anemia)
• Preparing for an organ transplant
• Forensic-criminalistic questions

Blood group: importance in transfusion medicine

If a patient is accidentally given a transfusion that is not compatible with AB0, this can have serious consequences (as described above): The red blood cells supplied are destroyed (intravascular hemolysis), which in the worst case leads to organ failure and death. Other possible complications of intolerance are:

• Malaise and nausea
• Sweats
• Skin spots and itching up to anaphylactic shock
• Circulatory collapse with subsequent kidney failure
• Shortness of breath

In the case of organ transplants, too, the doctor must be meticulous to ensure that the blood groups of the organ donor and organ recipient match. Otherwise there is a risk that the donor organ will be rejected in the new body. In exceptional cases, however, a special pre-treatment can make an AB0-incompatible organ transplant possible.

Which blood groups go together?

Due to the serious consequences of an incorrect blood transfusion, it is very important in transfusion medicine to carefully determine the blood groups of the donor blood and the recipient. In the case of red cell concentrates (EK), the following "pairs" are considered suitable:

Patient blood group	A.	B.	AWAY	0
EK blood group	A or 0	B or 0	AB, A, B or 0	0

Patients with blood group AB do not have antibodies against other blood groups and can receive all possible red cell concentrates. That is why this blood group is called the universal recipient.

In contrast, people with blood group 0 have no antigens on their erythrocytes, i.e. nothing to which other antibodies could react - they are referred to as universal donors: every patient can receive a transfusion with blood group 0 regardless of their blood group.

What is the bedside test?

With the bedside test, the doctor checks the blood group characteristics of a patient again before a blood transfusion in order to be sure to rule out any mix-ups. To do this, he takes a few drops of blood from the patient. He gives this to a special test field on which the antiserum is applied. If antigens come into contact with antibodies directed against them, the blood clots. If the blood groups match, however, the blood transfusion can be given.

Blood group intolerance in mother and child

An intolerance (incompatibility) in the AB0 system can become a problem during childbirth. However, since only antibodies from mothers with blood group 0 can cross the placenta, this blood group incompatibility only occurs between mothers with blood group 0 and children with blood groups A and B. In contrast to Rhesus incompatibility, it is not that severe. The most common consequence is jaundice in the newborn (jaundice), which may have to be treated with radiation with blue light (phototherapy).

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