Karl Landsteiner: The Map of Our Blood (A, B, O) (1930)

Vienna, 1900. A city of steaming cafés, waltzes in grand halls, and shadows stretching under gas lamps. In a small laboratory at the Pathological Institute, a thin man with a fine mustache and piercing eyes watches in frustration as another patient dies after a transfusion. His name is Karl Landsteiner, and that night, as he meticulously writes in his notebook, he doesn’t yet know he’s about to unravel one of the deepest mysteries of the human body: why does blood sometimes save lives and other times destroy them?

The scene repeats itself over and over. In 1875, a German doctor, Leonard Landois, had documented something terrifying: when he mixed blood from two different animals in a test tube, the red blood cells clumped together like rotten grapes, forming dark clots. But the worst was when he tried the same with humans. Some patients improved instantly after receiving new blood; others suffered chills, fever, and, in the worst cases, died with their kidneys blocked by those same clots. It was as if blood had a secret code that only some could decipher.

Landsteiner, obsessed with order and patterns, decides to investigate. In 1901, he gathers six colleagues from the lab: Dr. Pletschnig, Dr. Sturli, and four others whose names history almost forgets. He asks them to draw blood from one another, like guinea pigs. With glass pipettes, he mixes drops of blood on porcelain plates and observes. What he discovers leaves him breathless: not all human blood is the same.

• Sturli’s blood makes Pletschnig’s cells clump together like magnets.
• But Landsteiner’s blood doesn’t react with Sturli’s.
• And another colleague, Dr. Zar, has blood that doesn’t mix well with any of the others.

In just a few weeks, Landsteiner identifies three types of blood. He calls them A, B, and C (later, C would be renamed O). But there’s a fourth type, rarer, which he discovers years later: AB. It’s the first map of human blood, a system of compatibilities that will save millions of lives.

Yet the world doesn’t listen right away. In 1907, an American surgeon, Reuben Ottenberg, reads Landsteiner’s papers and performs the first successful transfusion using this system. But even then, many doctors prefer to rely on luck or archaic methods, like transfusing animal blood. Until World War I breaks out in 1914, and field hospitals fill with bleeding soldiers. How is it possible that something as vital as blood has such strict rules? And why did nature design this system of incompatibilities?