ABO blood group

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 antigens.

table of blood groups, red blood cell type, antibodies present in plasma and antigens present on red cells

From early childhood, normal healthy individuals make red cell antibodies against the A and/or B antigens that are not expressed in their own cells. These naturally occurring antibodies are known as isohaemagglutinins.

Isohaemagglutinins are mainly IgM immunoglobulins but can also be IgG. Anti-A isohaemagglutinins reacts with red cells of Group A or AB, and anti-B isohaemagglutinins react against red cells of Group B or AB. They bind to red cells carrying the corresponding antigen and trigger an immune response, causing the red cells to haemolyse.

If ABO incompatible red cells are transfused, immediate red cell haemolysis can occur.

For example, if Group A red cells are transfused into a Group O recipient, the recipient’s anti-A isohaemagglutinins bind to the transfused red cells.

An ABO incompatible transfusion reaction is an acute haemolytic transfusion reaction. This reaction can lead to overwhelming complement activation and result in shock, renal failure and disseminated intravascular coagulopathy (DIC).

If an ABO incompatible transfusion reaction is suspected, stopping the transfusion immediately may be life-saving.

ABO inheritance patterns 

Blood groups are inherited from our parents in the same way as other genetic traits (e.g. eye colour). 

The ABO blood group system is determined by the ABO gene, which is found on chromosome 9. The four ABO blood groups, A, B, AB and O, arise from inheriting one or more of the alternative forms of this gene (or alleles) namely A, B or O. 


Genetic combinations of ABO blood groups 

Blood group Possible genes
A AA or AO
B BB or BO

The A and B alleles are codominant so both A and B antigens will be expressed on the red cells whenever either allele is present. O alleles do not produce either A or B antigens, and are sometimes called ‘silent' alleles.  

ABO inheritance patterns 

Below are the possible blood groups that children may inherit according to the combination of parental blood groups. 

Parental blood groups Child's blood group
O and O O
O and A O or A
O and B O or B
O and AB A or B
A and A A or O
A and B O or A or B or AB
A and AB A or B or AB
B and B O or B
B and AB B or A or AB
AB and AB A or B or AB