Ehrlich, Paul (1854–1915), a German bacteriologist and chemist. For his research in immunology he shared the 1908 Nobel Prize in physiology or medicine with Élie Metchnikoff. Ehrlich developed a technique for staining tissues and cells for microscopic study. He introduced the side-chain theory of immunity, which explains how the blood produces antibodies against infectious organisms. After testing 605 different compounds, he discovered Salvarsan, or “606,” an arsenic compound for treatment of syphilis. Ehrlich was born in Silesia. He became professor at the University of Berlin and later director of the Royal Prussian Institute for Experimental Therapy.