Blood Group and Rhesus Factor – All You Need to Know

The Biomarker Handbook is a curated series that seeks to provide readers with insights on each biomarker we cover in our blood test packages and its relation to our body.

The human blood grouping is classified according to the existence of specific markers (antigens) found on red blood cells and in the plasma that permit the body to recognise blood as its own. In 1900, scientist Karl Landsteiner identified the A, B, and O blood types for which he was awarded the Nobel Prize in Physiology or Medicine in 1930. The fourth group AB was discovered by Alfred von Decastello and Adriano Sturli in 1902. Therefore, the ABO blood group system consists of A, B, AB, and O blood types.

  1. Group A has the A antigen in red blood cells and B antibody in the plasma.
  2. Group B has the B antigen in red blood cells and A antibody in the plasma.
  3. Group AB has both A and B antigens in red blood cells and no antibody in the plasma, so people who have group AB blood are called universal recipients because they are able to receive blood from a person with any other blood type.
  4. Group O has no antigens in red blood cells, but people in this group have A and B antibodies in the plasma so they are called universal donors because their blood can be donated to people with any of the ABO types.

Each person has two alleles in their blood group gene (one from the father, and the other one from the mother). The O allele is recessive to both A and B alleles.  As a result, people who have AO and BO genotypes will have A and B phenotypes respectively.  People who have O blood have OO genotypes.  In fact, they inherited a recessive O allele from both parents. If an A is inherited from one parent and a B from the other, the phenotype will be AB because A and B alleles are co-dominant. Agglutination tests will show that these individuals have the characteristics of both type A and type B blood.

Mixing blood from two people with different blood types can be fatal because the person receiving the blood has antibodies which will bind with opposing antigens in the red blood cells of the donor, causing a serious reaction. In conclusion, people with blood group A can safely get group A blood or group O blood, and people with blood group B can receive group B blood or group O blood. People with blood group AB can safely receive blood from all four blood groups. People with blood group O can only receive group O blood.

Group  A Group   B Group   AB Group   O
Antibodies in plasma Anti-B Anti-A None Anti-B and Anti-A
Antigens in red blood cells A antigen B Antigen A and B antigens None
Genotype AA or AO BB or BO AB OO

Rh Blood Group System

Based on the presence or absence of Rh antigen (the most significant Rh antigen is RhD) on the surface of the red blood cells, people are classified as Rh-positive (Rh+) or Rh-negative (Rh). If your blood is Rh+, you can get Rh+ or Rh blood transfusions. However, if your blood is Rh, you should only receive a transfusion from Rh donors except in extreme emergencies. This is attributed to the fact that Rh+ blood transfusion to Rh person can illicit the immune system of Rh person to make antibodies against the Rh antigen, leading to a transfusion reaction.

Rh + Rh
Blood group A A+  (AA+ or AO+) A  (AA or AO)
Blood group B B+  (BB+ or BO+) B  (BB or BO)
Blood group AB AB+ AB
Blood group O OO+ OO

Rhesus Incompatibility Between Mother and Foetus

It is one of the most classical examples of Rh incompatibility which occurs when an Rh mother is pregnant with a baby who is Rh+. In this case, the immune system of the mother will produce antibodies that are specifically designed to find and destroy these foreign blood cells, which have Rh antigens.

During the first pregnancy there is less chance of this happening as it takes time for the mother to produce these antibodies (sensitisation of her immune system). However note that the woman may have had a past miscarriage, and possibly been sensitised already.  Once the Rh positive cells of the next baby are recognised, the antibodies are programmed to attack the erythrocytes of the baby, leading to severe haemolytic disease of the newborn or erythroblastosis foetalis, which can be fatal to the foetus due to the rapid destruction of the erythrocytes of the foetus, and the foetus will not receive adequate oxygen.

In this case, the anaemic foetus’ body will try to synthesise erythrocytes rapidly, resulting in enlargement of liver and spleen. Also, these rapidly-produced erythrocytes are usually immature, and they are not yet able to perform their function properly.

Rapid destruction of the erythrocytes leads to high levels of bilirubin, which is a waste product of red blood cell breakdown. High levels of bilirubin will lead to jaundice and can lead to brain damage or heart failure.

Medical Intervention

Whenever there is Rhesus incompatibility (mother Rh- and foetus Rh+), the mother is given immunoglobulin (RhoGAM) to prevent incompatibility reaction. Prevention is the best form of management. It is common practice for the ABO and Rhesus testing to be performed during the first prenatal visit.

For newborns, the treatment depends on the severity of incompatibility reaction and may include: blood transfusion, intravenous fluids, management of respiratory difficulties, and use of intravenous immunoglobulin (to reduce red blood cell breakdown and levels of circulating bilirubin). If mild, the jaundice is treated with phototherapy.


In conclusion, after studying ABO and Rh group systems, we know that there are eight main blood types. The Rh-positive types are A+, B+, AB+ and O+. The Rh-negative blood types are A B, AB and O. Mismatches with the ABO and Rh blood types are responsible for the most serious, sometimes life-threatening, transfusion reactions.

If you want to find out more about why you need to know your ABO and rhesus factor, take a look at our lifestyle article here!

Interested in other biomarkers? Check out the rest of The Biomarker Handbook.

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