Who Discovered Blood Types and How Humans have different types of blood that are not all compatible?

Blood was blood, or so the world thought. Then Austrian physician Karl Landsteiner discovered that there were four types of blood.

Some could be safely mixed and some could not. That discovery has saved millions of lives. The day that Karl Landsteiner’s results were published, blood transfusions became a safe and risk-free part of surgery. A patient’s chances of surviving surgical procedures greatly increased. By making surgery safer, he made many new surgical procedures possible and practical.

Landsteiner’s discovery also greatly advanced human understanding of blood structure and blood chemistry and paved the way for a number of key medical discoveries in the early twentieth century.

Vienna, Austria, was a glamorous city in 1897, as modern as any in the world. Dr. Karl Landsteiner worked in the University of Vienna hospital, where he conducted cause-of-death (post mortem) medical examinations.

One April day that year, Landsteiner examined four patients who had died during surgery. All died for the same reason: blood agglutination (clotting). Each patient had received blood transfusions and died when his or her own red blood cells clumped together with red blood cells in the blood they were given into thick clots.

Landsteiner had seen this often during his thousands of post mortem examinations and wondered why it only happened with some patients.

That evening, Landsteiner played piano for his wife and several friends. It was the one thing Karl felt he did well. Most who heard him thought he should give up medicine for a brilliant career as a pianist.

In the middle of a familiar piece, it suddenly occurred to Landsteiner that the answer had to be something in the patients’ blood. What if all blood was not the same, as everyone supposed?

The next morning Landsteiner collected blood from 20 patients, wanting to see if he could predict which samples were safe to mix with each other.

In long rows of test tubes, he mixed a few drops of each patient’s blood with a few drops of blood from every other patient.

In his microscope, he checked to see which red blood cells clumped together, and which did not. Before he had checked half the test tubes under a microscope, Karl was stunned to find that he could easily divide the blood samples into two distinct groups. Red blood cells from any member of one group agglutinated (stuck to) red blood cells from every member of the other group. But the cells never stuck to blood cells of other members from the same group.

He named these groups “A” and “B.” Not all blood was compatible. Different people’s blood was different.

He continued testing and found blood samples that didn’t agglutinate with either type “A” or “B” red blood cells. Landsteiner realized that there must be a third group. People in this group could safely donate blood to anyone. He named this third blood group type “O.”

Then he found one blood sample that agglutinated with both type A and type B blood. There existed a fourth type of blood that reacted to both A and B blood, just as type O blood reacted to neither.
Karl named this fourth group type “AB.”

Blood was not all the same. There were four distinct types. Safe transfusions required a doctor to determine the blood types of both patient and donor. It seemed like such a simple, obvious idea, and yet is one that has saved millions of lives.

Humans have four blood types (A, B, AB, and O). Cats have the same number of possible blood types. Cows, however, have over