268 Chapter IX 



Pfeiffer and his collaborators, as well as many other investi- 

 gators, laid special stress on the bactericidal character of these 

 protective fluids. It was seen that the serums of immunised ani- 

 mals were often almost or completely incapable of killing the 

 corresponding micro-organisms, but they were still regarded as 

 bactericidal, because, when injected into the peritoneal cavity of 

 normal animals, they set up the transformation of vibrios into 

 granules, or, in the case of other bacteria, determined certain phe- 

 nomena of extracellular destruction. Whilst carrying on researches 

 in this direction, Frankel and Sobernheim 1 discovered a fact of great 

 importance. They found that the protective substance of the serum 

 of animals vaccinated against the vibrios resisted heating to 70 C. 

 When submitted to the influence of this temperature, the serum lost 

 its bactericidal power completely, but remained quite as protective 

 as the unheated serum, when injected into susceptible animals. This 

 experiment, which has since been confirmed repeatedly, furnished 

 us with a means of separating the bactericidal power from the 

 [282] protective power in cases where both were present in the same 

 serum. Later, in the hands of Bordet, it proved to be of great 

 service in connection with his researches on the concurrence of two 

 substances in acquired immunity. 



The possibility of obtaining Pfeifler's phenomenon outside the body 

 by "reactivating" the protective serum with peritoneal fluid or blood 

 serum of normal unvaccinated animals has still further facilitated 

 the study of the action of the two substances in acquired immunity. 

 It was with the help of this method that Bordet was able to furnish 

 so much valuable information on the subject of anti-cholera serums 

 and, later, on that of haemolytic serums. The discovery by Ehrlich 

 and Morgenroth 2 of the fixation by the sensitive elements of the 

 heat-resisting (thermostabile) substance (that which resists a tem- 

 perature of 65 70 C.) constitutes a new and important acquisition 

 to the study of acquired immunity. The discovery has been applied 

 by Bordet to micro-organisms, and since then it has been found 

 possible to study much more precisely the mode of action of 

 specific protective serums. 



Even before this last scientific advance had been made it was pos- 

 sible to determine the relations between the protective power and the 

 agglutinative power of the fluids of animals that had acquired 



1 Hygien. Rundschau, Berlin, 1894, iv Jahrg., SS. 97, 145. 



2 Berl klin. Wchnschr., 1899, S. 6. 



