Paul Ehrlich (1854-1915), German bacteriologist and Nobel laureate, noted for studies of the immune system and his method of treating syphilis.

Ehrlich was born in Strehlen, Silesia (now Strzelin, Poland), on March 14, 1854. He received a somewhat varied formal education in medicine, studying first at the University of Breslau (Wrocław), then the University of Strasbourg, returning to Breslau, and spending a final term at Leipzig, where his doctoral dissertation on the theory and practice of histological staining was approved in 1878. As head physician at the renowned Charité Hospital in Berlin, he advanced the field of hematology by developing methods of detecting and differentiating among various blood diseases. Most significant among his innovations was the use of different dyes, for example, methylene blue and indophenol blue, as selective vital stains for different types of cells.

Ehrlich's chief contribution to medicine was his side-chain theory of immunity, which established the chemical basis for the specificity of the immune response. The side-chain theory was an attempt to account for the ability of certain toxins both to produce a toxic effect and to elicit a specific immune response in mammals. Ehrlich postulated that cells have specific receptor molecules, or side chains, on their surfaces that bind only to certain chemical groups in toxin molecules; if the cells survive this binding they produce side chains in excess, some of them being released as circulating antitoxins, or what today would be called antibodies, in the blood. This theory laid the foundation for modern theories of immunity. Ehrlich also made great contributions in the field of chemotherapy, including using “606,” the so-called magic bullet known as Salvarsan, or organic arsenic, to treat syphilis.

Ehrlich shared the 1908 Nobel Prize in physiology or medicine with the Russian bacteriologist Elie Metchnikoff in recognition of their work on the chemistry of immunity. He also received the Prussian Great Gold Medal of Science (1903) and the Liebig Medal (1911), held honorary and foreign memberships in more than 80 scientific and medical societies, and received several honorary degrees. Ehrlich died in Hamburg, on August 20, 1915.