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figuratively speaking, that an umbrella against drizzle could be as effective in a downpour. In turning to this puzzle a century later, Ehrlich brought to the table his own specialty: great scientific rigor. With his ricin experiments, he developed a scrupulous methodology, juggling multiple factors. As a result, he knew exactly how much of the toxin was required to elicit immunity, according to a specific dosing schedule over a certain number of days. Likewise, he knew what amounts were too little or more than necessary. When his supermice had offspring, Ehrlich discovered something else of significance. The antitoxins were being passed on by the mother, from placenta to fetus as well as through suckling—textbook examples of passive immunization, in which an individual receives antibodies from another. (In active immunization, by contrast, protective antibodies are generated by one’s own immune system.)

Ehrlich’s methods and precision caught the attention of his peers, one of whom, fellow Berliner Dr. Emil von Behring, had just made his own startling discovery regarding passive immunity. In experiments completed in 1890, Behring found that if he removed the serum (the plasma without the blood cells and clotting elements) from an animal that had been successfully immunized against diphtheria, and then injected it into a second animal, that animal would also be immune. Serum from the injected animal would, in turn, protect other animals. Taking the next step, however—creating a diphtheria antitoxin to protect human beings—had proved troublesome. Behring wisely enlisted Ehrlich’s help in developing a safe, effective protocol. Ultimately, full-scale production of the lifesaving diphtheria treatment began in November 1894.

Paul Ehrlich in his laboratory

From there, a five-year jump in time finds a world-recognized Paul Ehrlich as the head of his own institute, the newly established Royal Institute of Experimental Therapy, located in Frankfurt—a long distance, both geographically and professionally, from his cramped quarters in Berlin. The institute had been designed to Ehrlich’s every specification, with multiple laboratories, a library, and ample space for a top-notch staff plus countless lab animals, all housed within a grand four-story building. Ehrlich oversaw a broad range of work whose scope was comparable, for its time, to, say, the United States’ National Institutes of Health combined with the Food and Drug Administration. While its opening ceremony in early November 1899 was a splendid public affair, attended by scientists, journalists, politicians, and citizenry, for Dr. Ehrlich personally a much more prestigious, albeit quieter, event would take place four months later.

It is March 22, 1900, and the forty-six-year-old Paul Ehrlich stands before the Royal Society of London—the exclusive scientific association that counts Antoni van Leeuwenhoek and Sir Isaac Newton among its past members. When he speaks of the great privilege it is to be here, this is no mere nicety. He has been invited to this first gathering of the new century to deliver the keynote address, a lecture titled “On Immunity with Special Reference to Cell Life.” He does not disappoint. In this now-legendary speech Dr. Ehrlich elaborates for the first time his “side-chain theory” of immunity, which provides a full accounting of the blood’s ability to protect the body from foreign invaders. Drawing upon the work of peers as well as his experience with ricin and diphtheria, Ehrlich explains that blood cells have on their surfaces ready-made receptor molecules, or “side chains,” that link or bind chemically with certain invading toxin molecules. (He had borrowed the term side chain from organic chemistry; it was widely believed that side chains were, like docking ports, the means by which cells took in nourishment from free-floating food particles.) Long story short, this binding neutralizes the toxins. It also triggers production of excess side chains, which are released into the blood as circulating antitoxins to fight the same toxin in the future.