206 SPECIFICITY OF AGGLUTININS 



species. Thus typhoid serum clumps B. coli, the paratyphoid and 

 paracolon bacilli, the B. psittacosis, and others. This is called a 

 group reaction, and is of profound interest in classification. It is 

 not, as might be thought, a hindrance to the practical application 

 of the process as a method of identification of the nature of a 

 culture, since it is found that the action is exerted much more 

 strongly on the organism used for the immunization than on 

 others. This is determined by ascertaining the dilution necessary 

 to bring about agglutination in a certain time at a given tempera- 

 ture. For example, we may find that certain specimens of anti- 

 typhoid serum will agglutinate typhoid bacilli at a dilution of 

 i : 10,000 in one hour, whilst B. coli is not affected if the dilution 

 is greater than i : 50. In the practical use of this serum we 

 should not be certain that a given culture was one of B. typhosus 

 unless it reacted at i : 1,000 or more. 



The explanation of these group reactions on Ehrlich's theory 

 offers no difficulties. Agglutinin is, as will be shown, a specific 

 antibody to the molecules of protoplasm contained in the bodies of 

 the injected cells. In each cell these will be of many varieties, 

 and to each a specific antibody will be produced. We must 

 imagine a typhoid bacillus as containing a large number of one 

 particular sort of molecule, a smaller one of another, whilst in the 

 colon bacillus these relations will be the reverse. A typhoid serum, 

 therefore, will contain much agglutinin which acts on the typhoid 

 molecules, and a little which acts on a few of those present in 

 B. coli ; it will agglutinate the former strongly, the latter feebly. 

 But the colon serum will contain antibodies to a few only of the 

 molecules present in the typhoid bacilli, and will clump it only in 

 strong dilutions. 1 



Agglutinins are formed, as we have seen, as the result of the 



1 There are a few noteworthy exceptions which have been recorded to these 

 general rules. In a few cases of tuberculosis the power of agglutinating 

 B. typhosus has been seen to rise, and Park has quoted a case in which an 

 animal immunized against staphylococci increased _its power against the same 

 bacillus from i : 10 to i : 160. In interpreting these results we must always 

 wonder whether they might not be explained by a rise in the sensitiveness of 

 the culture used. But this objection does not apply to the observations of 

 Posselt and Sagasser, who obtained an agglutinin which acted on bacteria 

 other than those used for the injection, and which was not removed from the 

 serum by these bacteria. And some cases have been recorded in which a 

 serum had less action on its own antigen than on others. All these exceptions 

 are rare and not fully investigated, and do not affect the general law. 



