In which branch of biology did Belgian microbiologist Jules Bordet specialize?

Jules Jean Baptiste Vincent Bordet (3 June 1870 – 6 April 1961) was a Belgian immunologist and microbiologist. The bacterial genus Bordetella is named after him. The Nobel Prize in Physiology or Medicine was awarded to him in 1919 for his discoveries relating to immunity.

In 1895 Bordet made his discovery that the bacteriolytic effect of acquired specific antibody is significantly enhanced in vivo by the presence of innate serum components which he termed alexine (but which are now known as complement). Four years later, in 1899, he described a similar destructive process involving complement, "hemolysis", in which foreign red blood cells are ruptured or "lysed" following exposure to immune serum. In 1900, he left Paris to found the Pasteur Institute in Brussels but continued to work extensively on the mechanisms involved in the action of complement. These studies became the basis for complement-fixation testing methods that enabled the development of serological tests for syphilis (specifically, the development of the Wassermann test by August von Wassermann). The same technique is used today in serologic testing for countless other diseases.

With Octave Gengou, he isolated Bordetella pertussis in pure culture in 1906 and posited it as the cause of whooping cough. He became Professor of Bacteriology at the Université Libre de Bruxelles in 1907.