Jules Bordet (1870-1961), the Belgian bacteriologist, was awarded the 1919 Nobel Prize for physiology (medicine). He won the prize for his discovery of a bactericidal factor in mammalium blood serum, now known as complement, and for determining that complement binds with antigen-antibody complexes - so-called complement fixation - to produce antibody-caused immunity.
This type of reaction became the basis for several diagnostic tests, such as the Wassermann test for syphilis. Bordet also discovered the bacillus that causes whooping cough and developed a vaccine against the disease.