There are several key aspects to consider when choosing the right blood for transfusion, but the challenges associated with blood types don't end there. (Image credit: ER Productions Limited via Getty Images)
More than a century ago, Dr. Karl Landsteiner discovered that red blood cells are not all the same. He noticed that mixing blood from different people often caused the cells to clot and break down. His research into the different types of red blood cells led to the ABO blood group system, which earned him the Nobel Prize in Physiology or Medicine in 1930 and made blood transfusions a safe and reliable practice that saves millions of lives each year.
Many people are familiar with their place in the classic ABO blood group system, which classifies blood into types A, B, AB, or O. However, these four main blood types are not a complete list.
So how many blood types are there really?
Depending on the depth of your research, there may be hundreds or more answers – and the list continues to expand.
Blood types exist because the red blood cells that carry oxygen in the body have different proteins and sugars on their surface, known as antigens. Each person has a unique combination of antigens on the surface of their blood cells, and this combination determines their blood type.
The ABO blood group system, developed by Landsteiner, classifies human blood according to the presence of one, both, or absence of antigens known as A and B. ABO blood type is determined by a single gene, one copy of which is passed on from each parent.
O is the most common version of this gene, but it is recessive, so in order to have blood type O, a person must inherit two copies. Both versions of the gene, A and B, are dominant, so they suppress any copy of the O gene. Having one copy of A or B will result in blood type A or B, respectively, and if both are present, the blood type will be AB.
“The ABO blood group system is the most critical blood type to consider when transfusing,” Dr. Emily Coberly, chief medical officer at the Red Cross, told Live Science in an email. “We all make antibodies against ABO antigens that are not present on our own red blood cells.”
For example, if you have blood type A, you will have antibodies, or protective immune proteins, that attack B antigens. So you shouldn't get type B blood. (This handy graphic from the Red Cross illustrates this concept.)
You may also have heard that blood types are “positive” and “negative.” This classification refers to another antigen known as the Rh factor. People with the Rh factor in their cells have Rh positive blood, while those without the Rh factor have Rh negative blood. The Rh factor is regulated by a cluster of genes that affect several antigens, so the genetics are more complex than the ABO system, but Rh positivity is both a dominant trait and more common in the population.
Different combinations of the ABO system and the Rh factor create eight main blood groups. In most cases, knowing which of these blood groups a person belongs to is enough to ensure a safe transfusion.
However, certain diseases can complicate matters, and blood types become even more confusing when you consider the hundreds of other antigens found on the surface of red blood cells.
What other blood types exist?
In addition to the A, B, and Rh factor antigens, there are at least 350 other known antigens on red blood cells, Coberly said. That number continues to grow as scientists identify important new proteins. If cells produce even one of these antigens differently than other cells, it justifies the creation of a new, unique blood type. So theoretically, there are as many blood types as there are combinations of surface antigens on a red blood cell.
As of 2024, the International Society of Blood Transfusion recognizes 47 blood group systems. Each of these systems can include multiple blood groups, just as the ABO system includes A, B, AB, and O.
Coberly cited examples of rare blood types, such as the MacLeod phenotype, in which a person's red blood cells do not express the Kx protein.
Sourse: www.livescience.com
