The Mystery Landsteiner Solved in 1901
For decades before 1901, early blood transfusions were essentially a gamble — some patients recovered, and others suffered severe reactions, with no clear pattern to explain why. Austrian scientist Karl Landsteiner solved the puzzle by mixing blood samples from his colleagues and noting which combinations caused the red blood cells to clump together, a reaction called agglutination. He identified three blood groups; a fourth, AB, was identified shortly after by two of his colleagues. Landsteiner had discovered the ABO blood group system, and the reason it mattered was antigens — marker molecules on the surface of red blood cells. Type A blood carries the A antigen, type B carries the B antigen, type AB carries both, and type O carries neither. The discovery earned Landsteiner the 1930 Nobel Prize in Physiology or Medicine and finally explained why some transfusions had always worked while others, mixing incompatible types, had been quietly killing patients.
Blood type is inherited in a straightforward Mendelian pattern from three possible alleles — A, B, and O — with A and B both dominant over O and equally expressed over each other when paired together, which is why a person who inherits one A allele and one B allele ends up with type AB blood rather than a blend of the two. A second, unrelated discovery followed in 1940, when Landsteiner and Alexander Wiener identified the Rh factor, a separate antigen named after the rhesus monkeys used in the research that revealed it. Roughly 85 percent of people are Rh positive, carrying the antigen; the remaining 15 percent are Rh negative.
Why O Negative and AB Positive Get Special Titles
Combine the four ABO types with the two Rh types and you get eight standard blood types, each with different transfusion compatibility. O negative carries none of the A, B, or Rh antigens, which means it triggers no antigen-based immune reaction regardless of who receives it — earning it the nickname "universal donor," especially valuable in emergency transfusions where there's no time to type a patient's blood first. AB positive works the opposite way: because it already carries all three antigens, a person with AB positive blood can receive red blood cells from any of the eight types without their immune system recognizing anything as foreign, making it the "universal recipient."
Blood itself is more than just type, though — it's a mix of red blood cells, white blood cells, platelets, and plasma, each doing distinct work. Red blood cells, which survive about 120 days before being broken down, mostly by the spleen, carry oxygen via hemoglobin. White blood cells, produced alongside red cells and platelets through a bone marrow process called hematopoiesis, defend against infection, with five distinct types — neutrophils, lymphocytes, monocytes, eosinophils, and basophils — each specializing in a different kind of threat. Platelets, really just fragments of larger bone marrow cells called megakaryocytes, gather at the site of any injury to a blood vessel and form the initial plug that starts the clotting process. An average adult carries about 5 liters of this mixture — a system Landsteiner's original slide of clumped and unclumped blood samples helped make safe to share between people for the first time.
Source: Encyclopaedia Britannica and the National Institutes of Health (NIH).