INFECTION AND IMMUNITY. 609 



sue-changes " underlying the establishment of acquired 

 immunity. 1 



The observations serving as the basis for this doctrine 

 have given to the blood and fluids of the body a new 

 and peculiar interest. According to circumstances, there 

 may be detected in the blood and tissue-juices a number 

 of bodies having totally different functions and affinities, 

 and therefore presumably different from one another. 

 To summarize briefly : First, there is normally present 

 in the blood-serum of practically all animals the de- 

 fensive "alexins" already mentioned. Second, the 

 antitoxins that are found in the blood of animals arti- 

 ficially immunized from special sorts of infection and 

 intoxication, as well as occasionally in the blood and 

 tissues of normal animals, the functions of which are 

 susceptible of demonstration outside the body as well as 

 within the tissues of the living animal. Third, a body 

 possessed of disintegrating, bacteriolytic powers, a bac- 

 teriolysin /. r., having the property of actually dissolving 

 bacteria, so that the phenomenon may be observed under 

 the microscope. This phenomenon is especially to be seen 

 within the peritoneum of guinea-pigs that have been ren- 

 dered immune from Asiatic cholera and from the typhoid 

 and colon infections or intoxications. 2 It is not to be 

 confounded with the ordinary bactericidal function of 

 the alexins that is demonstrable in most normal serums. 

 Fourth, a body, the so-called " agglutinin " (Gruber), 

 that, was considered by Widal to represent a " reaction 

 of infection/ 7 and not of immunity ; though at this time 

 its presence is generally supposed to indicate an effort 



1 Justice cannot be done to the beauty and ingenuity of this con- 

 ception in so brief a summary as is appropriate to a text- book. To be 

 appreciated it must be read as it came from its authors. 



2 It is generally known as Pfeiffer's phenomenon, 



39 



