Biologist Karl Landsteiner classified people and their blood into three groups, now known as A, B and O, with a fourth group AB discovered soon after. He showed that to successfully transfuse blood, matching blood types was crucial: transfusions between compatible groups were safe, but mixing different types could be deadly. 

Landsteiner was awarded the Nobel Prize in Physiology or Medicine 1930 for his discovery. During the rest of his career, Landsteiner continued investigating blood groups. In the 1940s, decades after his first discovery, he and collaborators discovered another blood group: Rh. Today, we know of over 200 minor blood groups, but the ABO and Rh systems are most important for determining blood compatibility. 

Read more about safe blood transfusions: nobelprize.org/stories/findin…