Charles Richet: When Our Defenses Turn Against Us (1913)

Boulogne-sur-Mer, 1902. The August sun beat down on the harbor, but inside the floating laboratory of the Princesse Alice II, a yacht turned research center, the air was icy. Charles Richet, a man with a thick mustache and piercing gaze, watched intently as his colleague Paul Portier injected a tiny dose of sea anemone venom into a dog named Neptune. "We just want to see how he reacts," Portier murmured, adjusting his fogged-up glasses.

Neptune, a grayish mixed-breed dog, wagged his tail indifferently. Minutes later, there was no sign of pain. Richet jotted in his notebook: "Initial dose tolerated." But what happened the next day left them breathless. When Portier injected an even smaller amount of the same venom, Neptune collapsed. His legs trembled, his breathing became a death rattle, and in less than five minutes, the dog lay dead. Richet stared at the body, his heart pounding. "This makes no sense," he whispered. "How can a smaller dose kill when the first one did nothing?"

That question would haunt him for years. Richet, a man of many talents—physiologist, writer, even playwright—had stumbled upon something that defied everything medicine thought it knew about the human body. This wasn’t the first time science had encountered this mystery. In 1839, French physician François Magendie had described how a dog that survived a dose of snake venom died after receiving a second, even smaller dose. In 1894, bacteriologist Emil von Behring, a future Nobel laureate, had noticed that some animals died when re-exposed to diphtheria toxins. But no one had connected the dots. No one, until Richet.

The most chilling case reached his ears in 1905. A young woman named Jeanne, 22, was admitted to Saint-Louis Hospital in Paris with a red rash covering her body. Doctors diagnosed hives, but when they administered a rabies serum—a common practice at the time—her body rebelled. Her throat swelled shut, her blood pressure plummeted, and she died within hours. The autopsy showed no sign of infection or poison. Only one detail stood out: Jeanne had received a dose of rabies serum a year earlier. Could her own body have killed her?

Richet began collecting stories like this. A child who died after eating a single egg, though he’d tolerated them before. A man who nearly lost his life after taking an aspirin, when he’d taken it without issue in the past. Each case followed the same terrifying pattern: the first exposure to the substance did nothing. The second, even in tiny doses, could be lethal. "It’s as if the body learns to hate," Richet wrote in his journal. But how? And why?

In his lab, Richet and Portier repeated the experiment with Neptune over and over, using different substances: jellyfish venom, horse serum, even egg proteins. The results were always the same. Something in the animals’—and humans’—bodies changed after the first exposure. Something made them hypersensitive. But what was that "something"? And how on earth could a defense mechanism turn into a deadly weapon?