146 ANTIGENS AND THE.TECHNIC OF SERUM REACTIONS 



about a typical reaction. 1 This and similar observations have 

 attracted attention to the similarity between the precipitation of 

 bacteria to which agglutinin is anchored by neutral salts, and the 

 precipitation of finely suspended 'clay by the addition of neutral salts; 

 the inference has been drawn that the phenomenon of agglutination 



one of physico-chemistry. 



Specificity of Agglutination Reactions: Group Agglutinins. The 

 composition of the agglutinogen that constituent of the bacterium 

 which stimulates agglutinin formation is unknown, but it appears 

 to be complex and probably not a single chemical compound. Closely 

 related bacteria may possess in common a small amount of agglu- 

 tinogen a least common multiple, as it were which stimulates the 

 production of "group agglutinin" that reacts with related bacteria 

 more or less in proportion to their content of the common antigen or 

 agglutinogen. . The specific agglutinin produced by the entire agglu- 

 tinogen content of an organism is more potent and fails to react with 

 related bacteria. Thus, the serum of an animal immunized against 

 B. typhosus may agglutinate that organism in a dilution of 1 to 3000; 

 B. paratyphosus will be agglutinated in a dilution of 1 to 300 by the 

 same serum, and B. coli would agglutinate only in a dilution of 1 to 

 50. The group agglutinin in this example would be effective for B. 

 paratyphosus in a dilution of 1 to 300, but in greater dilutions it would 

 be ineffective. For B. coli in the instance cited, the group agglutinin 

 is ineffective in dilutions above 1 to 50. 



The common or group agglutinin for B. paratyphosus in this typhoid 

 serum could be quantitatively removed by leaving it in contact with 

 a large number of paratyphoid bacilli for a few hours, then centrifu- 

 galizing to remove the organisms. The residual serum would contain 

 only agglutinin specific for B. typhosus. If B. typhosus were added 

 to the serum, all the agglutinin both "group" and specific would 

 be removed. 



As a general rule, group agglutinins constitute a minor fraction of 

 the total agglutinin and in practice the degree of dilution of the serum 

 used in specific cases is ample to exclude error. It occasionally happens 

 that sera of low dilution, especially those rich in agglutinoids, fail to 

 clump the specific organism; as the serum is diluted more and more 

 the phenomena of clumping become more and more marked; finally 

 a degree of dilution is reached beyond which the serum again becomes 

 ineffective. The initial negative agglutination in concentrated serum 



1 Bordet, Ann. Inst. Past., 1899, xiii, 225. 



