What makes your blood type A, B, AB or O?

Your blood type depends on which molecular tags, called antigens, sit on the surface of your red blood cells, and you inherit them from both your parents. In the ABO system, cells can carry antigen A (type A), antigen B (type B), both (type AB), or neither (type O). Add the Rh system, which gives the plus or minus, and you get the familiar types like A+ or O−.

Those tags matter because your blood also carries antibodies against the tags you don't have. When an antibody meets its matching antigen, the cells clump together. That's why transfusions have to be matched: incompatible blood can trigger a severe reaction that destroys red blood cells and can be fatal. It's also why O-negative donors are often called universal donors, and AB-positive people universal recipients.

We owe this to Karl Landsteiner, an Austrian physician in Vienna. In 1900, he noticed that blood from different people clumped when mixed in test tubes, and mentioned it in a two-sentence footnote. The next year he sorted human blood into three groups, which he called A, B and C. The C was later renamed O, from the German 'ohne', meaning 'without'. His discovery earned him the 1930 Nobel Prize, and World Blood Donor Day is now celebrated on his birthday.