INFECTION AND IMMUNITY. 573 



the inicroscoj)e. 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 tyjjhoid 

 and colon infections or intoxications.' 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," and not of immunity ; though at this time 

 its presence is generally supposed to indicate an effort 

 on the part of tlae body to resist infection. The pres- 

 ence of this body in a serum of an animal is announced 

 by its peculiar influence on the activity and arrangement 

 of the particular species of bacteria from which the indi- 

 vidual is immune, or with which it is infected. In the 

 case of typhoid fever in man, for instance, the serum 

 obtained during the early and middle stages of the dis- 

 ease, when mixed with fluid cultures or suspensions of 

 the typhoid bacillus, causes the bacilli to lose their 

 motility and to congregate (agglutinate) in masses and 

 clumps, a condition never seen in normal cultures of tiiis 

 organism, and practically never observed when normal 

 serum is employed. There are evidences of the pres- 

 ence of " agglutinin " in certain of the antitoxic serums 

 from artificially immunized animals, viz., that of ani- 

 mals immune from cholera, pyocyaneus, typhoid, dysen- 

 tery, and colon infections. So far as experience has 

 gone, this agglutinating property is manifested in the 

 great majority of cases only upon the particular organ- 

 isms from which the animal supplying the serum is 

 protected; that is to say, the relation is specific. In 

 view of the fact that the power of a serum to agglutinate 

 1 It is generally known as Pfeiffer's phenomenon. 



