Blood Types: ABO System, Frequencies, Significance

What are blood groups?

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

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

Blood Group Antibodies

How many blood groups are there in the AB0 system?

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

Blood group


Blood group: Antibody

Blood group A

AA or A0


Blood group B

BB or B0


Blood group AB



Blood group 0


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 carries this factor on his erythrocytes, he is Rh-positive. If the factor is absent, he 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. In Germany, it is found in only about five percent of the population. Overall, the blood group frequency in Germany is as follows:

AB0 and Rh blood groups (Germany)

Blood group A positive


Blood group A negative


Blood group B positive


Blood group B negative


Blood group 0 positive


Blood group 0 negative


Blood group AB positive


Blood group AB negative


When is the blood group determined?

The blood group is determined in the following cases:

  • Preventive care during pregnancy and for newborns
  • Preparation of an emergency card
  • Preparation of a blood transfusion, for example before an operation or in the case of severe anemia
  • Preparation of an organ transplant
  • Forensic-criminalistic questions

Blood group: significance in transfusion medicine

If a patient is inadvertently given a transfusion that is not AB0-compatible, this can have serious consequences (as described above): Destruction of the supplied erythrocytes (intravascular hemolysis) occurs, which in the worst case leads to organ failure and death. Other possible complications of intolerance are:

  • malaise and nausea
  • @ Sweating
  • Circulatory collapse with subsequent kidney failure
  • Respiratory distress

In the case of organ transplants, the doctor must also take great care to ensure that the blood groups of the organ donor and the organ recipient match. Otherwise, there is a risk that the donor organ will be rejected in the new body. In exceptional cases, however, special pretreatment can make AB0-incompatible organ transplantation possible.

Which blood groups are compatible?

Because of the serious consequences of incorrect blood transfusion, it is very important in transfusion medicine to carefully determine the blood groups of the donor blood and the recipient. For red blood cell (RBC) concentrates, the following “pairs” are considered to match:

Patient blood group





EC Blood Group

A or 0

B or 0

AB, A, B or 0


Patients with blood group AB have no antibodies against other blood groups and can receive all possible red cell concentrates. Therefore, this blood group is called a universal recipient.

What is the bedside test?

With the bedside test, the doctor checks the blood group characteristics of a patient once again before a blood transfusion in order to rule out a mix-up with absolute certainty. To do this, he takes a few drops of blood from the patient. This is then placed on a special test field to which antiserum has been applied. If antigens come into contact with antibodies directed against them, the blood clumps together. However, if the blood groups match, the blood transfusion can be administered.

Blood group incompatibility in mother & child