VACCINATION AGAINST TYPHOID 375 



injecting a bacterium, not only may agglutinins capable of acting 

 on that bacterium appear in the serum, but the serum may 

 become capable of agglutinating other, and especially kindred, 

 bacteria ; further, any normal agglutinins for the infecting 

 bacterium present in the serum may be increased in amount. 

 The agglutinin acting on the infecting organism has been called 

 the primary or homologous agglutinin, while the others have 

 been called the secondary or heterologous agglutinins. But 

 besides what we know to be a fact, that infection by a single 

 bacillary species can originate agglutinins acting both on itself 

 and on allied species, we must consider the possibility of 

 infections by more than one species occurring in an animal, e.g. 

 b. typhosus with b. coli or with b. paratyphosus (vide infra). In 

 such a case each organism may originate its primary agglutinin, 

 so that the presence of multiple agglutinins in a serum may 

 really be an indication of a mixed infection. Some attention 

 has been directed to the diagnosis and differentiation of these 

 conditions. Castellani introduced the absorption method for 

 their investigation (for method, see p. 121), and by this means 

 studied the primary and secondary agglutinins produced in 

 infections in rabbits ; he found that when an animal had been 

 infected with b. typhosus this organism would absorb from its 

 serum not only the primary typhoid agglutinins but also such 

 secondary agglutinins as those acting on the b. coli. If, how- 

 ever, an animal had undergone infection with, say, both the 

 b. typhosus and the b. coli, then the b. typhosus could not absorb 

 from its serum the b. coli (primary) agglutiuin. Castellani thus 

 put forward the view that by this means primary could be 

 differentiated from secondary agglutinins, and therefore pure 

 could be differentiated from mixed infections. There is little 

 doubt that this view possesses considerable validity, though it is 

 probably not of universal applicability. Safe deductions can 

 only be drawn when any serum is tested with several species of 

 fairly closely related organisms, such as those of the coli group. 

 Especially is it necessary that the highest dilutions in which 

 agglutination occurs should be compared. If such precautions 

 be adopted, the absorption method can be utilised for the differ- 

 entiation of the typhoid and paratyphoid organisms and their 

 infections, and for similar investigations. 



Vaccination against Typhoid. The principles of the im- 

 munisation of animals against typhoid bacilli have been applied 

 by Wright and Semple to man in the following way : Typhoid 

 bacilli are obtained of such virulence that a quarter of a twenty- 

 four hours' old sloped agar culture when administered hypo- 



