1. Karl Landsteiner's 1900 Discovery Hidden in a Footnote
In 1900, Karl Landsteiner published a paper on a different subject. It is introduced that the footnote of that paper contained an observation: serum from healthy people not only clumped animal red blood cells but sometimes clumped red blood cells from other humans as well [1].
Serum is the liquid left after blood clots, containing no red blood cells. When red blood cells gather into clumps, it is called "agglutination." If you ask what year the blood type classification was announced, sources may say 1900 or 1901. This difference depends on whether you count the footnote observation or the classification paper published the next year [2]. This article treats 1900 as the footnote observation and 1901 as the announcement of the classification. So, how did Landsteiner verify that single line?
2. Landsteiner's 1901 Experiment Mixing Blood from 22 People into Three Groups
In 1901, Landsteiner published a paper in a medical journal in Vienna. It is described that he took serum and red blood cells from 22 healthy people, including himself and colleagues, mixed the combinations, and checked if they clumped [2].
The results showed that human blood could be divided into three types: A, B, and C. At this stage, the type C is the one that would later be called "O" [3]. The AB type was not included in this classification. The year after the footnote, the classification was announced based on the experiment with 22 people. So, who found the fourth type and when?
3. Von Decastello and Sturli Find the Fourth Blood Type, AB, in 1902
The fourth type is said to have been found in 1902 by Alfred von Decastello, a colleague of Landsteiner, and his student Adriano Sturli [4]. This is the AB type.
Some summaries of explanatory articles suggested that American researchers added the AB type. However, looking at English-language sources, it is stated that the two people in Vienna in 1902 were the ones who found it [5]. With this, all four types were identified, but the naming conventions were still inconsistent.
4. Why the Blood Type Called "C" Became Known as "O"
In 1910, it is said that von Dungern and Hirschfeld popularized the naming where Landsteiner’s C was called the number 0 (zero), and the fourth type was called AB [5].
At that time, two naming systems using letters and numbers existed side by side, which confused readers. In 1927, Landsteiner himself proposed replacing the two systems with the letters O, A, B, and AB, and it is said this naming continues today [5]. So, what kind of confusion was the "number system"?
5. Why Roman Numeral Blood Type Names Meant Different Types in Different Countries
In 1907, Czech serologist Jansky classified the four types using Roman numerals I through IV. Type I corresponded to the current O, II to A, III to B, and IV to AB [5].
In the same year, American physician Moss created a classification using the same numbers but with a different correspondence. In Moss’s system, I was AB, II was A, III was B, and IV was O. Types I and IV were reversed [5]. In 1907, Ottenberg, a physician at Mount Sinai Hospital in New York, is said to have performed one of the first blood transfusions after checking blood types [8]. This means that when blood types began to be used for transfusions, the same numbers referred to different types. In 1921, it is said that American societies recommended adopting the Jansky system [5]. Names and numbers were gradually organized. So, how are blood types passed from parents to children?
6. How the Inheritance of Blood Types Was Corrected in 1924
In 1910, Dungern and Hirschfeld considered two pairs of genes: "A or not A" and "B or not B." However, with this idea, it was possible for an O-type child to be born from AB-type parents [6].
In 1924, Felix Bernstein corrected the explanation to one involving three alleles: A, B, and O [6]. Alleles are different versions of a gene located at the same place. It is said that this explanation is still used today. Landsteiner received the Nobel Prize in Physiology or Medicine in 1930, about 30 years after the classification paper of 1901 [7].
7. Using Paper Cards A, B and O to Understand Blood Type Inheritance
You can check the rules of inheritance using three types of cards labeled A, B, and O. A person receives one card from each of their parents. If you have A and B, you are AB type. If you have two O cards, you are O type. If you have A and O, you are treated as A type; if B and O, as B type.
Consider an AB-type parent (A and B) and an O-type parent (O and O). Write out the combinations the child might receive and check if an O-type child can be born. It can be verified that an O-type child cannot be born from an AB-type parent and an O-type parent.
If you want to look at the original texts, the initial footnote observation was published in the paper in Centralblatt für Bakteriologie 27:357 in 1900. The 1901 paper that wrote the classification can also be found in the references.