90 INFECTION AND RESISTANCE 



ceived the process as a physical phenomenon in which the bacteria 

 themselves were entirely passive, and, indeed, Widal 40 soon demon- 

 strated that bacteria killed by heat were equally as agglutinable as 

 the living germs. 



This naturally suggested that the reaction between specific agglu- 

 tinating serum and bacteria was based on individual peculiarities of 

 the bacterial proteins, and it occurred to Kraus, 41 accordingly, to 

 investigate whether or not the immune sera would cause any sort of 

 reaction when mixed with the dissolved body substances of homolo- 

 gous bacteria. Working at first with cholera and plague, he pre- 

 pared solutions of bacterial proteins, both by allowing broth cultures 

 to stand for varying periods and by emulsifying agar cultures in 

 alkaline broth. The extracts were then filtered through Pukal filters 

 to remove the bacterial bodies. When the sera of immunized ani- 

 mals were added to these clear filtrates cholera serum to cholera 

 filtrate, and plague serum to plague filtrate, slight turbidity devel- 

 oped and was followed within twenty-four hours by the formation of 

 small flakes. In other words, it was found that the mixture of a 

 clear filtrate of a bacterial culture with the serum of an animal 

 immunized against these bacteria resulted in the formation of a 

 precipitate. The reaction was found to be as strictly specific as that 

 of agglutination. 



Although, from the beginning, Paltauf 42 attempted to associate 

 the phenomena of agglutination and precipitation, the property of 

 precipitating homologous culture filtrates was attributed by Kraus 

 and others to specific antibodies in the immune sera, distinct and 

 independent of those previously described, and spoke of them as 

 "preciptiins." 



The discovery of the various "antibodies" so far discussed re- 

 sulted from the study of the direct action of blood serum upon bac- 

 teria and bacterial products. This did not, however, completely de- 

 flect the attention of investigators from the unquestionable impor- 

 tance of phagocytosis in the defence of animals against bacterial in- 

 vasion. Metchnikoff and his school continued diligently to pursue 

 this other phase of the study of immunity and, although the increas- 

 ing knowledge of serum antibodies continued to strengthen the prem- 

 ises of the purely humoral point of view, it had still to be admitted 

 that in some diseases particularly anthrax and the pyogenic coccus 

 infections, phagocytosis must largely be held responsible for recov- 

 ery. It was found, moreover, by the later investigations of Denys, 

 Wright, Neufeld, and others that phagocytosis in immunized animals 

 was far more extensive and efficient than in normal ones, and that 



40 Widal. La semaine medicale, No. 5, 1897. 



41 Kraus. Wien. klin. Woch., No. 32, 1897. 



42 Paltauf. "Discussion of Kraus' Paper," Wien. kl Woch., No. 18, 1897. 

 p. 431. 



