In 1901, the Viennese pathologist Karl Landsteiner discovered that centuries of attempted blood transfusions had failed because practitioners had overlooked one simple factor: that blood falls into distinct groups.

Landsteiner discovered different types of protein and sugar markers - known as antigens - on the surface of people's red blood cells. He realised that blood transfusions between people with different types of antigens failed because the body's immune system would attack the foreign red blood cells.

Landsteiner went on to classify the blood of humans into the now well-known A, B, AB and O groups, allowing safe blood transfusion on a mass scale. So he created the ABO blood group system that we still use today. 

In 1930 he was awarded the Nobel Prize for Medicine.

Source: Al Jazeera