The Two-Sentence Footnote
In 1900, an Austrian doctor named Karl Landsteiner mixed blood from different people in test tubes and noticed something strange. Sometimes the blood clumped together. He mentioned it in a footnote just two sentences long.
That footnote was the first evidence that human blood is not all the same. The next year, he showed that one person's blood would clump only with blood from certain other people, and he sorted blood into three groups: A, B and C. Group C was later renamed O, after the German word ohne, meaning without. His students found a fourth group, later named AB, a year after that.
Landsteiner won the Nobel Prize in Physiology or Medicine in 1930 for the discovery.
The early years were messy. Two doctors created rival numbering systems that gave the same numbers to different groups, causing confusion and real danger in hospitals. The letters O, A, B and AB were adopted internationally in 1928.
What Your Blood Type Actually Means
Your blood type depends on markers sitting on the surface of your red blood cells. Each marker is called an antigen, and you inherit them from both of your parents.
The most important system is ABO. Group A has the A antigen, group B has the B antigen, group AB has both, and group O has neither. Your blood also carries a matching antibody for any of these antigens you lack, a protein that attacks cells carrying that marker. So group A blood carries antibodies against B, and group O carries antibodies against both A and B.
The second key system is Rh. If your red blood cells carry the D antigen, you are Rh positive, the plus sign in a type like A+. If they do not, you are Rh negative. Rh negative blood is much more common in Europe, at about 15 percent of people, than in Asia, at about 0.3 percent.
ABO and Rh are just the beginning. As of June 2025, 48 blood group systems were officially recognised, and many are named after the patients in whom their antibodies were first found.
Who Can Give Blood to Whom
Getting the match wrong is dangerous. If a patient receives incompatible blood in a transfusion, their antibodies attack the new red blood cells. The result can be a severe reaction with kidney failure and shock, and death is possible.
That leads to some simple rules. Group O red blood cells have no A or B antigens, so they can be given to people of any ABO group. Someone with O negative blood is often called a universal donor. Group AB blood has no antibodies against A or B, so people with AB can receive red blood cells from any group, and AB positive people are called universal recipients.
There is a catch. Because O negative works for almost anyone, it is often overused and is always in short supply. Experts say it should be saved for O negative patients, women who might be pregnant, and true emergencies.
For plasma, the liquid part of blood, the rules flip: AB plasma can go to anyone, while O plasma only suits O patients. To be extra safe, blood banks can cross-match a patient's blood against a donor's first.
Mothers, Babies and a Personality Myth
Blood types matter in pregnancy too. If an Rh negative mother carries an Rh positive baby, some of the baby's blood cells can pass into her bloodstream, often around birth. Her body may then make antibodies that attack the red blood cells of that baby or a later one. This is called hemolytic disease of the newborn, and it can be fatal.
One of the great medical advances of the 20th century was a way to prevent it: an injection that stops the mother from making those antibodies in the first place.
Your blood type almost never changes, with one surprising exception. After a bone marrow transplant from a donor with a different ABO type, a patient's blood type should eventually become the donor's.
Blood types have even crept into popular culture. In Japan and South Korea, many people believe your blood type predicts your personality and who you will get along with. Researchers have found no scientific basis for it.
