Blood types are defined by the presence or absence of certain antigens on the surface of red blood cells. The most well-known antigen groups in human blood are the ABH antigens and the Rhesus or Rh antigens. The ABH group has 4 blood types: A, B, AB, and O. So why is type O called type O instead of type C or type D? The story behind the naming of the ABO blood group sheds some interesting light on the history of blood type research.
The Discovery of ABO Blood Types
The ABO blood group was first discovered in 1900 by Austrian scientist Karl Landsteiner. Landsteiner found that when he mixed blood from different people, the red blood cells sometimes clumped together and sometimes didn’t. This clumping was caused by antigens on the blood cells reacting with antibodies in the serum. Based on these reactions, Landsteiner categorized blood into 3 groups which he called A, B, and C. A few years later, his students discovered a fourth type which Landsteiner named type O.
So why was the fourth blood type called type O? Basically, it was because type O red blood cells did not have either the A or B antigens. Type O blood was lacking both of the known blood group antigens at the time, so Landsteiner called it group O for “ohne”, the German word for without.
The Discovery of the Rhesus Factor
The next major development in the ABO blood group system came in 1937 when Karl Landsteiner and Alexander Wiener discovered the Rhesus or Rh blood group. The Rh group has dozens of different antigens, but the most significant is the D antigen. People who have the Rh D antigen are Rh positive, while those who lack it are Rh negative.
The Rh group is named after the Rhesus monkey which was used in early experiments. When rabbits were injected with Rhesus monkey blood, they produced antibodies against the Rh antigens. This demonstrated that the Rh antigens were foreign to rabbits, and therefore were likely foreign to any non-Rhesus animals, including humans. Further research revealed that around 85% of Europeans were Rh positive while 15% were Rh negative, confirming that the Rh antigens were not universal to all people.
Modern Understanding of ABO Blood Types
Today, the ABO blood group system is fully characterized at the molecular level. The A and B antigens are carbohydrate molecules attached to proteins and lipids on the surface of red blood cells. The antigens are produced by a series of enzymes that add various carbohydrates to a precursor substance. People with type A blood have the enzyme that adds N-acetylgalactosamine, people with type B have the enzyme for galactose, type AB have both enzymes, and type O have neither.
The O in type O blood stands for zero – no A or B antigens present. Here is a summary of the ABO blood groups:
| Blood Type | Antigens Present |
|---|---|
| A | A |
| B | B |
| AB | A and B |
| O | Neither A nor B |
The lack of A and B antigens in type O means that type O blood can be safely transfused into most other blood types. For this reason, type O is often called the “universal donor”. On the flip side, people with type O blood can only receive type O red blood cells because other types could provoke an immune reaction.
Other Blood Group Systems
Since Landsteiner’s early work, dozens of other blood group systems and hundreds of individual blood group antigens have been identified. Besides the ABO and Rh groups, some of the best known human blood group systems are:
- MNSs – Discovered in 1927, over 40 antigens
- Kell – Discovered in 1946, over 20 antigens
- Duffy – Discovered in 1950, 5 main antigens
- Kidd – Discovered in 1951, 3 main antigens
- Lewis – Discovered in 1946, 5 main antigens
Most of these minor blood group antigens are not as immunogenic as ABO and Rh, but they can still cause complications in blood transfusions and pregnancies. Their discovery expanded our knowledge of the diversity of human blood types.
Significance of the ABO System
Although other blood group systems exist, the ABO group remains the most important by far for blood transfusions. Determining a patient’s ABO type is a critical first step for safe blood transfusions. The ABO group is also still the only blood group routinely considered in organ transplantation.
Outside of transfusion medicine, numerous studies continue to uncover associations between ABO blood types and disease risks. People with certain blood types appear to have higher or lower risk for various medical conditions such as cancer, cardiovascular disease, and severe COVID-19. The reasons for these associations are still being investigated, but may relate to subtle differences in immune function between blood types.
Conclusion
In summary, the O in blood type O stands for an absence of A and B antigens. When Karl Landsteiner first discovered blood groups in 1900, he named the fourth blood type “O” to indicate that it lacked the A and B antigen “factors” that defined the other known blood types at the time. Although many other blood group systems have since been discovered, the century-old ABO system remains the foundation of blood banking and transfusion medicine.
The O antigen status gives type O blood special properties as a “universal donor” for transfusions. And associations between ABO types and disease continue to be uncovered. So while the O may have started out meaning “ohne” or “zero”, it turns out to be far from meaningless. The reason for its name reveals an important chapter in the biological understanding of blood.