When engaging an enemy in battle, it’s always an advantage to enlist some help, and in the case of the immune system this is no exception. To aid their vital task of specifically binding to and destroying invading bacteria and viruses, antibodies recruit a special type of protein to deliver a lethal blow. The identity and behaviour of this attack-boosting protein was revealed by Jules Bordet, for which he received the 1919 Nobel Prize in Physiology or Medicine.

Bordet built upon Richard Pfeifer’s studies, which revealed that an active agent in the abdominal cavities of guinea pigs killed cholera bacteria by bursting through their cell walls. Puzzlingly, these bacteria didn’t die when trying to replicate these conditions in the laboratory. Through a meticulous set of experiments in which he took different combinations of fresh, preserved or heated blood from animals infected with cholera and injected them into normal animals, Bordet concluded that the destruction of these bacteria relied on a team effort. Antibodies created and released into the blood stream to specifically attack bacteria required the presence of a heat-sensitive substance always present in blood, which was initially named alexin, and later given the more appropriate name of complement.

Bordet later discovered that this antibody/complement knockout punch is dished out in response to any invading foreign agent, as red blood cells from rabbits injected into guinea pigs suffered the same fate as bacteria. These observations were of enormous practical benefit to researchers examining the finer details of the immune response, as introducing foreign blood cells into animals provided a more controllable method for stimulating antibody production than injecting bacteria, which grew rapidly and unpredictably. Taking advantage of the specific way in which complement proteins bind to antibodies, Bordet also developed a medical test for detecting infectious agents in blood. This complement-fixation test became invaluable for diagnosing many diseases, most notably with the development of a modified form of the test for detecting syphilis.