of the humoral branch of the immune system Research on complement began in the 1890s, when Jules bordet at the institut pasteur in paris showed that sheep antiserum to the bacterium vibrio cholerae caused lysis of the bacteria and that heating the antiserum destroyed its bacteriolytic activity. Surprisingly, the ability to lyse the bacteria was restored to the heated serum by adding fresh serum that contained no antibodies directed a The Functions of Complement