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Many blood group systems have been identified, but for transfusion safety the two most important blood group systems are ABO and Rh.
Correctly identifying the blood groups of both blood donors and recipients is vital for ensuring that suitably compatible products are selected for transfusion.
The clinical importance of the various blood groups depends on their ability to stimulate antibodies that can destroy transfused red blood cells or cross the placenta and cause haemolytic disease of the fetus and newborn (HDFN).
This is dependent upon:
- the frequency with which antigens and antibodies occur, which may vary in different populations due to ethnic diversity
- the functional characteristics of antibodies such as thermal range, immunoglobulin class, titre, avidity and ability to fix complement
Updated April 2025
More about blood groups
ABO
ABO is the most important of all the blood group systems. There are four different ABO blood groups, determined by whether an individual's red cells carry the A antigen, the B antigen, both A and B antigens, or neither antigen.
Learn moreRh
Over 55 antigens have been described in the Rh system, with new antigens still being discovered.
Learn moreBlood group phenotypes
An individual’s blood group phenotype is determined by the expression of antigens on their red cells. The frequency of blood group phenotypes within a population is determined by the ethnic diversity of a region and the varying patterns of inheritance of the blood groups.
Learn moreRed cell phenotype calculator
This calculator allows the user to select a specific red cell phenotype from any combination of blood group systems and calculate the frequency of that combination in the general population.
Learn moreRed cell compatibility calculator
This calculator determines the number of ABO matched donor units a blood service or hospital transfusion laboratory would need to test to find compatible blood for a patient with red cell antibodies.
Learn more
