AGGLUTININS 4! 



characteristics mentioned, are regarded as entirely 

 identical or almost so, are sharply differentiated by 

 means of their agglutination. In other words, the 

 " groups," arrived at by means of a common agglu- 

 tination sometimes have no relation to species as the 

 term is usually employed. Thus, according to Stern, 

 certain varieties of proteus and of staphylococci ex- 

 cite the production of sera which exert marked agglu- 

 tinating powers also on typhoid bacilli, although 

 otherwise we do not regard these three micro- 

 organisms as at all related. On the other hand by 

 means of agglutination we can sharply distinguish 

 cholera bacilli from their nearest related species. 

 Because of this lack of absolute specificity the 

 serum diagnosis of infection or the identification of 

 bacteria has value only when very carefully tested. 

 Absorption Methods for Differentiating between 

 a Mixed and a Single Injection. In 1902, Castellani 1 

 called attention to a procedure which consists in 

 saturating the diluted immune serum with succes- 

 sive quantities of the bacteria most strongly agglu- 

 tinated until the agglutinating power for these 

 bacteria = o. After centrifuging the mixture the 

 clear fluid is tested on the second variety of bacteria, 

 and from this one learns whether mixed or single 

 infection was present. According to Castellani if 

 the serum of an animal immunized against a certain 

 microorganism is saturated with that organism, 



1 Castellani, Zeitschrift Hygiene, Vol. xl, 1902. 



