Bordet, when working in Pasteur’S laboratory in Paris in 1896, observed that serum of animals immunized with bacteria (Vibrio cholerae), could agglutinate and lyse those bacteria in vitro (either in a test tube or on a slide). If, however, the serum was left at room temperature for one week, or briefly heated at 60 °C, the agglutinating activity remained unchanged but the ability to lyse bacteria was lost. The lytic activity could be restored by adding fresh serum from a non-immunized animal to the bacteria-heated serum mixture. On the basis of this experiment, Bordet assumed that lysis of bacteria required the presence of two factors; specific, agglutinating antibodies (discovered by Behring in 1889) and a non-specific component that was activated secondarily and was responsible for bacterial lysis and killing. Bordet called the latter component “alexin”. Soon after this discovery, particularly when further data were obtained by Ehrlich, the non-specific serum component was renamed “complement”.