How blood groups are inherited and why it´s important that you know yours

Do you know your blood type? This is a question we should all be able to answer. It contains vital information for situations such as blood donations, transfusions or pregnancies. It is also very interesting to know how blood groups are inherited, a very common question asked in medical consultations. Do you want to know why? This post explains it all:

The relationship between red blood cells and blood types 

Blood is made up of different cell types suspended in a liquid called “plasma”. Blood is considered a tissue and consists mainly of red blood cells, platelets, and white blood cells. Blood types are determined by “antigens”, which are molecules found on the surface of red blood cells.

Each type of cell performs a different function:

• Platelets: these cells do not have a nucleus and are involved in blood clotting when a blood vessel has ruptured. 
• White blood cells: also called leukocytes, these cells are part of the body’s immune system, fighting infection and disease. 
• Red blood cells: also called erythrocytes, they give the blood its distinctive red colour and are responsible for transporting oxygen and carbon dioxide. The antigens that are located on the cell surface form the various blood groups. 

The two most important and well-known antigenic systems are the ABO system and the RH system.

The ABO blood group system

The so-called ABO system was discovered by the Austrian pathologist and biologist Karl Landsteiner  in 1901. Up until that point, some human to human blood transfusions had been proven to be successful and others not, but the reason was unknown.

Landsteiner noticed that when two people’s blood was mixed, it reacted in two ways: either by clumping together and clotting, or by fusing together. Consequently, he discovered three different types of red blood cell antigens: A, B and O, a finding for which he later received the Nobel Prize.

The ABO gene has three alleles: A, B and O, and the blood type is determined by the presence or absence of these 3 alleles. An allele is each of the forms in which the same gene can be expressed. As previously outlined, the different blood groups are determined by the absence or presence of certain antigens in the red blood cells. These antigens vary depending on what alleles they comprise. Therefore:

• Alleles A and B form type A and type B antigens respectively.
• Allele O does not produce any type of antigen.

Therefore, depending on the antigens that are present or absent in the red blood cells, we can identify 4 blood groups: Group A, Group B, Group AB and Group O. 

The Rh system or D antigen

Like the ABO system, the Rh (Rhesus) system, also called the D antigen, is based on the absence or presence of a certain antigen on the red blood cell membrane surface: 

• If this antigen is present, the blood cell will be Rh positive and will have no antibodies against this antigen, which is the most common scenario.
• If this antigen is absent, the blood cell will be Rh negative. This generally does not pose any problem for the individual’s health; it is only important for pregnant women. 

Blood group compatibility

Not all blood groups are compatible with each other, there is specific compatibility for both the ABO group and the Rh factor. The table below illustrates the compatibility of different blood groups for receiving and donating blood: 

Reference image from https://www.giveblood.ie/learn-about-blood/blood_group_basics/

A transfusion between non-compatible blood groups, both ABO and Rh groups, will generate antibodies that trigger what is called “acute haemolytic transfusion reaction”, which has serious health implications. This is because the immune reaction that is triggered causes the breakdown of red blood cells (haemolysis).

For example:

• Blood group A reacts with group B or AB.
• Blood group B reacts with group A or AB.
• Blood group O reacts with groups A, B or AB, but can be used for transfusions with all other groups. That is why people with type O blood are called “universal donors”. 
• Blood group AB does not react with groups A, B, AB or O.
• The Rh negative blood group reacts with the Rh positive blood group.
• The AB positive blood group does not react with any group, hence people with this blood group are called “universal recipient”.

Rh incompatibility in pregnancy 

It is extremely important to take a mother’s Rh factor into account during pregnancy. 

Rh positive is the most common blood group. Rh negative is less common, and certain precautions must be taken with Rh negative women when pregnant. If their partner is Rh positive, there is a possibility that the baby will also be Rh positive, which may lead to complications during pregnancy. This occurs if the blood of the mother and child come into contact.

Although the mother’s blood does not normally mix with the baby’s during pregnancy, it could happen during delivery. If this happens, the mother’s Rh negative blood can react with the child’s Rh antibodies. This is usually not a problem in a first pregnancy, but it can be a problem in later pregnancies, as antibodies could penetrate the placenta and attack the red blood cells of a future Rh-positive baby. This could cause anaemia in the foetus that could kill it.

At the first prenatal consultation, the doctor will request a blood group and Rh factor test for the mother. If you are Rh negative, you will probably have an antibody test in the first trimester to detect possible Rh positive antibodies and, if necessary, take further action with anti-D gamma globulin injections. This prevents the mother’s body from producing Rh antibodies throughout the pregnancy and after delivery.

Tests to determine blood group

A blood sample is needed to determine an individual’s blood group. There are two steps:

1st The sample is mixed with blood type A and type B antibodies and monitored to see whether the red blood cells in the blood clump together or not. 

If there are clots, this means that the blood has reacted with one of the antibodies.

2nd This is called reverse typing. Here, blood plasma (without cells) is mixed with type A or type B blood:

• People with type A blood have type B antibodies.
• People with type B blood have type A antibodies.
• People with type O blood have both type A and type B antibodies.

To establish whether the blood is Rh positive or negative, we look at whether or not the blood has proteins on the cell surface. If there are proteins on the cell surface, it would be Rh positive and if not, it would be Rh negative. 

How are blood groups inherited?

Blood groups are hereditary. They have a Mendelian pattern, in other words, they are the product of a single gene. 

The ABO gene has three types of alleles: A, B, and O. The first two, A and B, are codominant, in other words, they dominate equally. However, allele O is recessive.

The different combinations among these three alleles generate the different blood groups.

For example, a person with blood group AB has one A gene and one B gene, while a person with blood group A may have two AA genes or one A gene and one O gene, but since it is dominant, A prevails. 

The table below displays a parent-child group compatibility table:

Reference imagen from https://canadiancrc.com/paternity_determination_blood_type.aspx

As for the most common blood groups in Spain, those with a positive Rh factor are the most prominent, specifically A+, O+ and B+.

Are we close to “making” universal blood?

The scientist, Stephen G. Withers and his team, from the University of British Columbia , have outlined how to convert group A blood to O, by using two enzymes from the gut microbiota.  This finding, which was published in the Nature Microbiology journal in June of this year, if validated, would lead to an increased supply of the universal donor blood group.

This article has talked about how blood groups play a very important role in key processes, such as a blood transfusion or during pregnancy, and knowing them is essential for avoiding any possible complications.

Please note that blood from a universal donor is used in emergencies, but even so we highly recommend familiarising yourself with this important information. Do you know your blood group?