More than 100 years ago, Dr. Karl Landsteiner discovered that not all red blood cells are alike. He found that, when different people's blood was mixed, it would often clump up and curdle, destroying the cells within it. His work on the different types of red blood cells led to the discovery of the ABO blood group system, which won him the 1930 Nobel Prize in physiology or medicine and also helped make blood transfusion a safe, reliable practice that saves millions of lives every year.
Many people know where they fit into the classic ABO blood typing system, which designates blood as type A, B, AB or O. But these four basic blood groups aren't the end of the story.
So how many blood types are there, really?
Depending on how deep you dig, the answer could number in the hundreds or more — and the list is still growing.
Related: What's the rarest blood type?
Blood types exist because red blood cells, which relay oxygen through the bloodstream, carry different proteins and sugars on their surfaces. These are called antigens. Each person has a specific combination of antigens on the surfaces of their blood cells, and that combination determines their blood type.
Landsteiner's ABO blood group system categorizes a person's blood based on whether they have one, both or neither of the antigens known as A and B. ABO blood type is determined by a single gene, one copy of which is passed down by each parent.
O is the most common version of that gene — but it's recessive, so a person needs to inherit two copies to have an O blood type. Both the A and B versions of the gene are dominant, so they overpower any copy of the O gene. Having one copy of A or B will lead to an A or B blood type, respectively, and a person will have an AB blood type if both A and B are present.
"The ABO blood group system is the most important blood group to consider during a blood transfusion," Dr. Emily Coberly, divisional chief medical officer at the Red Cross, told Live Science in an email. "We all make antibodies against the ABO antigens that aren't on our own red blood cells."
For example, if you have type A blood, you'll have antibodies, or protective immune proteins, that attack B antigens. So you wouldn't want type B blood in your body. (This handy graphic from the Red Cross illustrates the idea.)
You also may have heard of blood types being "positive" or "negative." This categorization comes down to another antigen, called the Rh factor. People with the Rh factor on their cells have Rh-positive blood, whereas people without the Rh factor are Rh-negative. The Rh factor is controlled by a cluster of genes influencing multiple antigens, so the genetics are more complicated than in the ABO system, but being Rh-positive is both a dominant trait and more common in the population.
The various combinations of the ABO system and Rh factor create the eight main blood groups. In most cases, knowing which of these blood groups a person fits into is enough to provide them a safe transfusion.
But certain diseases can complicate things, and blood types get even more complex when you consider the hundreds of other antigens present on the surface of red blood cells.
What are the other blood types?
Besides the A, B and Rh factor antigens, there are at least 350 other known antigens on red blood cells, Coberly said. That number is still going up as researchers identify important new proteins. If cells build just one of these antigens differently than other cells do, that justifies labeling a new, unique blood type. So, theoretically, there are as many blood types as there are combinations of surface antigens on the red blood cell.
As of 2024, the International Society of Blood Transfusion recognizes 47 blood group systems. Each of these blood group systems can encompass multiple blood types, just as the ABO system includes A, B, AB and O.
Coberly said some examples of these rarer blood types include the McLeod phenotype, in which a person's red blood cells don't express a protein called Kx; the Kidd-null phenotype, in which cells don't express a group of proteins known as the Kidd group; and the Bombay phenotype, in which cells don't express a protein called the H antigen.
Like the ABO group and Rh factor, these rare blood types are genetic. They may also be associated with various health conditions and symptoms; for example, the McLeod phenotype is associated with the neurological disorder McLeod syndrome.. And because of the genetic component, they may be tied to certain racial or ethnic backgrounds.
For example, sickle cell disease (SCD), a condition that causes red blood cells to form in a crescent-moon shape, primarily affects people of African or Hispanic descent. People affected by SCD often need repeated blood transfusions, which can lead the immune systems to become sensitized to the donor blood. This causes about half of those treated for the disease to develop antibodies against the myriad antigens present on the donor blood cells. That happens even when the donated blood is matched to the patients' ABO type and Rh factor.
After developing antibodies, these patients require meticulously matched blood that accounts for these additional immune proteins. Otherwise, their immune systems may attack and destroy the donor blood, which can quickly lead to life-threatening complications.
These complexities in blood types are one reason why blood banks always need new donors, Coberly emphasized.
"This is why it is so important that we maintain a diverse blood supply from blood donors of all backgrounds," she said. "Blood will be available for all patients when they need it, including those that need blood that is more closely matched to them."
Some scientists are trying to bypass this issue entirely by creating universal donor blood — whether it's by growing red blood cells from scratch or removing antigens from existing donated blood.
