GROUP AGGLUTINATION. 225 



cally, are closely related to the homologous bac- 

 terium. In these instances, the agglutinating 

 power is greatest for the homologous organism, 

 and the degree to which the heterologous organisms 

 are agglutinated is, to some extent, an index of 

 the proximity of the relationship of the latter to 

 the former. Antityphoid serum has been found to 

 agglutinate the psittacosis, colon, paracolon, and 

 paratyphoid bacilli and Bacillus enteritidis, but 

 the action is never so strong as on the typhoid 

 bacillus itself. We are to understand that this 

 power to agglutinate related organisms represents 

 something more than the normal property of the 

 serum ; there has been an actual increase in agglu- 

 tinin for the heterologous bacteria as a result of 

 infection or immunization by the primary organ- 

 ism. 



Having typhoid fever in mind, this is a rule 

 which works both ways. Infections with the colon 

 bacillus and related organisms, and sometimes 

 with organisms not closely related, as the staphylo- 

 coccus, may cause an increase in agglutinin for the 

 typhoid bacillus. The importance of this fact is 

 evident, and it may explain the positive Gruber- 

 Widal reaction sometimes found in infections 

 other than typhoid. 



Inasmuch as the highest agglutinating power is 

 always manifest against the homologous organism, 

 this is spoken of as the chief agglutinin (Haupt- 

 agglutinin) of the serum, while the weaker agglu- 

 tinins for other organisms are called partial or ad- 

 ventitious agglutinins, or coagglutinins (Mitag- 

 glutinin) . 



The phenomenon of group agglutination would specificity. 

 seem to violate the specificity which we are in the 



