Article 006: The Man Who Discovered Blood Types & His Lifelong Search Through Human Blood Karl Landsteiner

Today, being asked your blood type seems completely ordinary. A, B, AB, O. Positive or negative. Those few letters and symbols can determine which blood you can safely receive, which blood you can donate, and sometimes how a pregnancy needs to be medically managed. But a little more than a century ago, doctors did not understand these differences. Blood looked like blood, and although physicians had experimented with transfusions for centuries, the results could be dangerously unpredictable. One patient might survive while another became severely ill or died. No one fully understood why.

Then an Austrian physician named Karl Landsteiner began mixing people's blood together.

Landsteiner was born in Vienna on June 14, 1868. He studied medicine at the University of Vienna and received his medical degree in 1891, but his interests quickly went beyond ordinary medical practice. He became deeply interested in chemistry, immunity, antibodies, and the subtle differences between biological materials. He spent several years working in chemistry laboratories before returning to Vienna, where he continued investigating some of the fundamental questions of human biology.

At the time, scientists already understood that blood from different animal species could react when mixed. Landsteiner began asking a much more interesting question: Could there also be important differences between the blood of two perfectly healthy human beings?

Around 1900, he began performing a remarkably simple experiment. Landsteiner collected blood from himself and several colleagues. He separated the red blood cells from the liquid serum and then systematically mixed the red cells of one person with serum from another. Some combinations mixed without an obvious reaction. Others caused the red blood cells to clump together, a reaction known as agglutination.

Most importantly, the reactions weren't random.

Landsteiner recognized that the samples could be divided into groups according to how they reacted with one another. In 1901, he published his findings and described three groups that he called A, B, and C. His C group later became known as O. Soon afterward, Alfred von Decastello and Adriano Sturli identified the fourth major group, AB, completing what developed into the familiar ABO blood-group system.

It was an extraordinary discovery because it revealed something that had been invisible throughout human history: not everyone's blood is biologically compatible with everyone else's blood.

Today we understand the mechanism much better. Red blood cells can carry specific molecules called antigens on their surfaces. In the ABO system, the most important are the A and B antigens. Plasma can contain antibodies that recognize incompatible antigens. When incompatible blood is transfused, those antibodies can attack the transfused red blood cells, potentially causing a dangerous and sometimes life-threatening transfusion reaction.

Landsteiner's experiments therefore helped explain why earlier attempts at human blood transfusion had produced such inconsistent results. The problem wasn't necessarily the concept of transferring blood from one human being to another. The problem was compatibility.

Interestingly, the medical importance of Landsteiner's discovery wasn't immediately appreciated. His findings were confirmed, but it took years for their enormous implications for transfusion medicine to become clear. As blood typing and crossmatching developed, physicians gained the ability to test donor and recipient blood before transfusion. What had once been partly a medical gamble could increasingly become a controlled scientific procedure.

But Landsteiner's story didn't end with ABO.

He continued researching blood, immunity, antibodies, and infectious disease for decades. His work with Erwin Popper helped demonstrate the infectious nature of poliomyelitis, and after Landsteiner eventually moved to the United States and joined the Rockefeller Institute for Medical Research in New York, his laboratory continued identifying previously unknown differences between human red blood cells. His work with other researchers contributed to the discovery and understanding of additional blood-group antigens, including the M, N, and P systems.

Then, almost forty years after his original ABO experiments, Landsteiner became involved in another discovery particularly important to anyone interested in Rh-negative blood.

In 1940, Karl Landsteiner and Alexander Wiener reported experiments involving antibodies produced after rabbits were exposed to red blood cells from rhesus macaques. This research became historically associated with what came to be called the Rh factor. The complete history is more complicated than simply saying Landsteiner “discovered Rh-negative blood,” however. Work by Philip Levine and Rufus Stetson involving an unusual antibody following pregnancy was also crucial to understanding the human Rh blood-group system, and researchers later learned that the human Rh system was considerably more complicated than the original rhesus-monkey experiments suggested.

That distinction is important because the Rh system isn't simply a positive-or-negative switch. Today we know it involves the RHD and RHCE genes and numerous antigens and genetic variants. The familiar positive or negative designation generally refers to whether the important D antigen is detected on someone's red blood cells. Variants such as weak D and partial D make the genetics even more interesting.

Landsteiner eventually received the world's highest scientific recognition for his earlier work. In 1930, nearly three decades after publishing his blood-group discovery, he received the Nobel Prize in Physiology or Medicine “for his discovery of human blood groups.”

By then, the simple observation he had made by mixing blood samples had helped transform medicine.

Landsteiner could have considered his life's work finished. He didn't.

He became emeritus at Rockefeller in 1939, when he was already in his seventies, but he continued going into the laboratory and conducting research. Accounts of his life describe a scientist who remained intensely interested in his work even in old age. Blood had occupied much of his scientific career, and he continued studying it almost until his final moments.

On June 24, 1943, at the age of 75, Karl Landsteiner suffered a heart attack while working in his laboratory at Rockefeller. He was taken to the Institute's hospital, where he died two days later on June 26, 1943. His Nobel biography describes the circumstances memorably: Landsteiner effectively died “pipette in hand.”

There is something fitting about that ending. More than forty years after his first blood-group experiments, the scientist who had revealed some of the hidden differences in human blood was still conducting research.

Landsteiner's discovery did much more than give humanity the letters A, B, AB, and O. It demonstrated that blood contains biological differences that cannot be seen simply by looking at it. Two people can appear completely healthy and yet possess blood that behaves very differently when brought together.

His work helped lay the foundation for modern blood typing, crossmatching, transfusion medicine, immunohematology, and later discoveries involving blood-group genetics. It also opened questions researchers are still investigating today about how different blood-group systems evolved, why their frequencies vary among human populations, and what selective pressures may have helped preserve particular variants throughout human history.

And perhaps the most remarkable part of the entire story is how the discovery began.

There was no genome sequencer. There was no massive computer database. Landsteiner simply separated blood into its components, mixed samples from different people, carefully observed what happened, and recognized that the pattern meant something.

The blood clumped. He asked why. And from that question, an entire hidden system within human blood began to reveal itself.