It is to the Austrian biologist and doctor Karl Landsteiner that we owe the discovery of the ABO system. It was in 1900. At the time, we already know that by mixing animal blood, red blood cells agglutinate and burst. Landsteiner observes that the same reaction can occur when human blood is mixed … but not always. This is how he decides to classify human bloods into groups.

It proves that blood transfusion is safe in people with blood of the same group. ABO, what exactly is that? “These are antigens – like labels – expressed on the surface of red blood cells,” explains Professor Jacques Chiaroni, hematologist, Director of the French blood establishment PACA / Corsica.

Focus on red blood cells

Red blood cells are the cells that carry oxygen in our body. “When you have label A, you are group A. If you have label B, you are group B. If you have labels A and B, you are group AB. And finally, if you have no label, you are O. So two labels define four blood groups. »In France, group A comes first: it is carried by 45% of the population, followed by group O (43%), and followed, far behind, by groups B (9%), and AB (only 3%), according to the French Blood Establishment.

We make antibodies against antigens that we don’t have. “When you are in group A, you make anti-B antibodies,” explains Professor Chiaroni. “Group B makes anti-A antibodies. Group O, which has no tag, makes anti-A and anti-B antibodies. The AB group does not manufacture anti-A or anti-B because it does not have a label.

It is on these antibodies that the rules of transfusion compatibility depend. If the recipient’s anti-A (or anti-B) antibodies bind to the A (or B) antigens of the donor’s red blood cells, they cause these cells to clump together or even destroy them. “At a minimum, the transfusion is ineffective. At worst, it can cause death. This is why compatibility during a transfusion must be respected.

After the letter, there is a + or a -. It has a name: the Rhesus system, now called RH. “In 1939, Levine and Stetson discovered the Rhesus factor while investigating a hemolytic disease of the newborn. This indicates the presence or absence of an antigen called “Rh D” in a protein of the membrane of the red blood cell. “The rhesus system is still a matter of etiquette. If you have the Rh D label, you are rh positive. If you don’t have it, you are rh negative. “

An Rh D- person can donate blood to an Rh D + person… but the reverse is not true. Someone Rh D + cannot give someone Rh D-. The combination of the two systems allows classification into eight main blood groups: A +, A-, B +, B-, AB +, AB-, O + and O-.

• The people of group A can donate blood to people in groups A and AB. They can receive blood from people in groups A and O. 
• The people of group B can donate blood to people in groups B and AB. They can accommodate groups B and O.
• “The people of group O- are universal donors because they have neither label A nor label B. In an emergency setting, they can donate blood to all groups. But they can only receive blood from people in group O. “
• “The universal receiver, AB +, has both labels for him, so he can receive everything (A, B, AB, O). But he can only donate blood to other people in the AB group. “

That said, except in an extreme emergency, a person will always receive blood from their own group.

“In total, there are 40 blood group systems, and almost 380 labels. Among these systems, some are rarer. A blood group is said to be rare in France when its frequency is less than 1 in 250, and there is no transfusion alternative.

There is for example the Bombay phenotype. Almost everyone on the planet has the H label except them. Bombay can only be transfused with Bombay, so as not to kill the patient. The frequency of Bombay in India is estimated at 1 / 10.000. Elsewhere in the world? It is even rarer.

“Group O is less susceptible to severe forms of malaria. On the other hand, it is more sensitive to cholera, while B is more resistant to it. The link between blood groups and infectious pathologies was established a long time ago. As for Covid 19, it again seems that we are not all equal in the face of the epidemic.

“Group O seems to be less susceptible to Covid 19, unlike group A, overrepresented,” reports Professor Chiaroni. A study by Danish researchers, published in the journal Blood Advances, points in this direction. One hypothesis: people in group O would be less likely to be infected thanks to their anti-A and anti-B antibodies which recognize these antigens which can be carried by the virus.

But beware: being in group O does not mean that you are protected from this virus at all. To protect oneself, and protect others, an imperative, again and again: respect barrier gestures.