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illuminated them, revealing contrast and texture, making microscopic specimens easier to analyze. Weigert introduced his cousin to this important advance, and Paul began experimenting on his own. In 1872 he went off to study medicine at the University of Breslau and, as was customary in those days, transferred to different schools every year to train with the finest teachers. At the University of Strasbourg, while under the tutelage of one of the great anatomists of the nineteenth century, Wilhelm Waldeyer, Paul invented a technique that took his cousin’s discovery one step farther: “selective staining.” Using a dye of his own formulation, he found that each cellular element in a tissue sample reacted differently to his staining and also displayed a different shade, thus permitting extraordinarily sharp definition—akin to what HDTV is to regular television, I imagine. With this method, Paul promptly made his first major discovery: the mast cell, a type of cell common in connective tissue.

Praise for his innovative staining was not unanimous, however. While completing his studies at the University of Leipzig, Paul lived at a boardinghouse whose owner would recall many years later that the young student often looked like a human drop cloth; his hands, face, and clothes were always spotted with inky stains. His bath towels and linens were equally blemished, and no amount of washing could remove the blight. Moreover, even the house’s billiard table, upon which, for lack of flat surfaces, Paul had conducted experiments, was forever splotched with fuchsia, indigo, and lilac. Little could the proprietor have known then that her boarder’s messes would lead to findings that would make a permanent impact on several branches of science. Ehrlich had not been content simply to observe the myriad cells of the human body—as dazzling as he found them—but sought to figure out why dyes became fixed in specimens (as they did in fabrics), plus why individual cellular parts reacted so differently to certain dyes. From controlled experiments, he concluded that, rather than a mere physical change, a specific chemical transformation was occurring within the cells. These investigations into the nature of staining were the subject of Ehrlich’s 1878 doctoral thesis and also anticipated the dawning of a new field of biology, cytochemistry, the study of cell constituents. The thesis also contained the germ of a larger theory explaining how—in lay terms—disparate substances bind chemically, which would evolve over the next thirty years into his ideas about the formation of antibodies in human blood; his concept of a magic bullet; and, ultimately, the invention of the syphilis cure. But that’s jumping ahead.

Already renowned for his histological staining, the newly minted doctor was invited in 1878 to join the staff of the prestigious Charité Hospital in Berlin, where he worked under the supervision of Friedrich von Frerichs, an esteemed clinician. Though Ehrlich had a full roster of patients, Dr. Frerichs recognized the young man’s talent and energy and encouraged him to spend more time on original research. Ehrlich continued experimenting with aniline dyes but turned his attention from animal tissue to human blood, which, in a clinical setting, was readily available. The study of blood, too, was still fresh terrain. Although two centuries had passed since Antoni van Leeuwenhoek’s discovery of red blood cells, progress in the field of hematology had been sluggish. From today’s vantage point, it appears that Ehrlich was the right scientist at the right time and place, equipped with the right tools, to transform the entire discipline. Ensconced in his closet-sized hospital lab, the twenty-four-year-old quickly invented a simple method for preparing blood specimens. Well, simple for him. First he would take great care to spread a small drop of blood on a glass slide in the thinnest possible layer. He then allowed it to air-dry. Next, as he reported in a published paper, he heated the blood smear on a copper hot plate “for one or several hours” at 120