When you're in need of a blood transfusion, you'll be given blood that matches the blood your own body produces. But this wasn't always the case. The reason we know about it is because of an Austrian and American biologist by the name of Karl Landsteiner, who discovered the blood groups in 1901.
Landsteiner, who died in June 1943, would have been 148 years old on June 14.
Prior to Landsteiner's discovery, blood transfusions had been attempted as early as the 17th century. Successful transfusions were conducted between animals, but when attempts were made to transfuse animal blood into humans, the human immune system rejected the blood. This can get fatal. The host's antibodies can attack the new red blood cells, breaking them down and causing blood to clot in the veins.
This can occur when a transfusion recipient is given incompatible human blood, too. While some successful human transfusions had taken place in the 19th century, it was largely by luck that they did succeed, and many did not.
When Landsteiner identified the A, B and O blood groups, he created a means for testing the compatibility between donor and recipient blood, making blood transfusions much safer. While receiving the wrong blood still does sometimes happen today, it is usually by accident, very rare, and medical staff are better equipped to deal with the fallout.
Landsteiner was awarded the Nobel Prize in 1930 for his discovery, but his achievements extended to other areas as well. He worked for over a decade on immunity and antibodies and, in collaboration with physician Erwin Popper, isolated the virus that causes polio in 1908, which in turn allowed the creation of a vaccine.