text: In the early 1890s, Paul Ehrlich started to work with Emil Behring, professor of medicine at the University of Marburg. Behring had been investigating antibacterial agents and discovered a diphtheria antitoxin (that is, antibodies that target a biological toxin produced by the diphtheria bacteria Corynebacterium diphtheriae). (For that discovery, Bering was the first recipient of the Nobel Prize in Physiology or Medicine in 1901. Ehrlich was also nominated for that year.) From Behring's work, Ehrlich understood that antibodies produced in the blood could attack invading pathogens without any harmful effect on the body. He speculated that these antibodies act as bullets fired from a gun to target specific microbes. But after further research, he realised that antibodies sometimes failed to kill microbes. This led him to abandon his first concept of the magic bullet.