How blood groups are inherited and why it is important that you know what yours is

Do you know what blood group you have? This is a question that we should all know how to answer, as it is essential information in situations such as blood donations, transfusions or pregnancies. In addition, it is also very interesting to know how blood groups are inherited , since it is a very common question in medical consultations. You want to know why? In this post we explain it to you.

The relationship between red blood cells and blood groups 

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

Each type of cell performs a different function:

  • Platelets: these cells without a nucleus are involved in blood clotting when there is a break in a blood vessel.
  • 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 red blood cells, they give blood its characteristic red color and are responsible for transporting oxygen and carbon dioxide. The antigens, present on the surface of these cells, give rise to the different blood groups. 

The two most important and well-known antigenic systems are the AB0 system and the RH system.

The AB0 blood group system

The so-called AB0 system was discovered by the Austrian pathologist and biologist Karl Landsteiner in 1901. Until then, it had been proven that some blood transfusions between humans were successful and others were not, however, the reason was unknown.

Landsteiner noticed that when the blood of two people was mixed, it reacted in two ways: either clumping together and forming clumps, or fusing. In this way, he discovered three different types of red blood cell antigens: A, B, and 0 , a finding for which he later received the Nobel Prize. 

The AB0 gene has three alleles: A, B and 0 and the blood group 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 we have previously explained, the different blood groups are given by the absence or presence of certain antigens in the red blood cells. These antigens are different according to the alleles that form them. So:

  • The A and B alleles form the type A and type B antigens respectively.
  • The 0 allele does not produce any type of antigen.

In this way, and depending on the antigens that are present or lacking in the red blood cells, we distinguish 4 blood groups: Group A, Group B, Group AB and Group 0.

The Rh system or D antigen

The Rh (Rhesus) System, also called D antigen, in the same way that the AB0 system is based on the absence or presence of a specific antigen on the membranous surface of red blood cells:

  • If this antigen is present , the blood cell will be Rh positive and will not have antibodies against this antigen, this is the most common situation.
  • If there is an absence of said antigen, the blood cell will be Rh negative . Generally it does not imply any problem for the health of the person, it is only important in women facing a pregnancy. 

Blood group compatibility

Not all blood groups are compatible with each other, there is a specific compatibility for both the AB0 group and the Rh factor. In the following table we show you the compatibility of the different blood groups when it comes to receiving and donating blood: 

If a transfusion is carried out between non-compatible blood groups, both in the AB0 and Rh groups, antibodies are generated that cause what is called an “acute hemolytic transfusion reaction” with serious consequences for health, since the immune reaction that is triggered causes the breakdown of red blood cells (hemolysis). 

For example:

  • Blood group A reacts against groups B or AB.
  • Blood group B reacts against groups A or AB.
  • Blood group 0 reacts against groups A, B or AB, but can be used for transfusions to all other groups. That is why people with type 0 blood are called “universal donors”. 
  • The AB blood group does not react to groups A, B, AB or 0.
  • Rh negative blood group reacts to Rh positive blood group. 
  • AB positive blood group does not react to any group, hence people with this blood group are called “universal receptors”.

Rh incompatibility in pregnancy 

During pregnancy it is very important to take into account the mother’s Rh factor.

Rh positive is the most common blood group. Rh negative is less common, and in the face of a pregnancy in an Rh negative woman it is necessary to take certain precautions. If the couple is Rh positive, there is a possibility that the baby is also Rh positive, in which case complications can occur during the pregnancy. This occurs if the blood of mother and child come into contact.

Although it is normal for the mother’s blood not to mix with the baby’s during pregnancy, it can happen during delivery. If this happens, the mother’s Rh-negative blood can react with the child’s Rh antibodies. This is not a problem in a first pregnancy, but it is a problem for subsequent pregnancies, since the antibodies could cross the placenta and attack the red blood cells of a future Rh-positive baby . In this case, it could cause anemia in the fetus that could cause her death.

In the first prenatal visit, the doctor will prescribe an analysis to find out the mother’s blood group and her Rh factor. If you are Rh negative, you will probably have an antibody test in the first trimester to detect possible Rh factor-positive antibodies and, if necessary, take appropriate measures by injecting Anti-D gamma globulin that prevents the mother’s body from generating Rh antibodies to throughout pregnancy and after delivery.

Tests to determine the blood group

To determine the blood group, a blood sample is necessary. The test takes place in two steps:

The sample is mixed with antibodies against type A and type B blood and it is checked whether the red blood cells agglutinate or not.

If lumps are observed, the blood has reacted with one of the antibodies.

It is called reverse test. Here the blood plasma (without cells) is mixed with type A or type B blood:

  • People with type A blood have antibodies against type B. 
  • People with type B blood have antibodies against type A. 
  • People with type 0 blood have antibodies to both type A and type B.

To determine Rh positive or negative , it is considered whether or not the blood has proteins on the cell surface. In the first case it would be Rh positive and in the second, negative. 

How are blood groups inherited?

Blood groups are hereditary. Their pattern is Mendelian, that is, to explain it simply, they are the consequence of a single gene. So how are blood groups inherited?

The AB0 gene has three types of alleles: A, B and 0. The first two, A and B, are codominant, that is, they dominate equally. However, the 0 allele is recessive.

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

For example, a person with blood group AB has one A and one B gene, while a person with A blood group may have two AA genes or one A gene and another 0, but A prevails, since it is dominant. 

In the following table we show you a compatibility table of parent-child groups:

Regarding the most frequent blood groups in Spain , those with a positive Rh factor predominate, specifically A+, 0+ and B+. 

Are you looking for a universal “manufacturer”?

The scientist, Stephen G. Withers and his team, from the University of British Columbia, have described how to convert blood from group A to 0, through the action of two enzymes of the gut microbiota. This finding, which was published in the journal Nature Microbiology in June this year, if validated, would increase the reserves of this blood group, which, as we saw before, is universal. 

As you have seen in this article, blood groups play a very important role in key processes, such as a blood transfusion or during pregnancy, and knowing them is important to avoid any type of complication.

It is important to point out that, in case of emergency, universal donor blood is used , but even so it is highly recommended to know such relevant information. And do you already know what blood group you have?

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