THE PHENOMENON OF AGGLUTINATION 231 



to.ooo 



ZONE OF 



ABSOLUTS 

 SPECIFICITY 



species. It is the specific antigen in response to which the "chief" 

 agglutinin is formed, while the others, present in smaller quantity, 

 lead to the formation of the minor or paraagglutinins with an in- 

 tensity proportionate to the amounts present in the bacterial cell. 

 Thus, as Durham expresses it, if we assume one micro-organism to 

 contain antigenic substances a, b, c, and d, and another d, e, f, and g, 

 the antibodies produced by injections of the former would react with 

 the common element d in the latter. 



The diagnostic value of the specificity, however, is plainly not 

 affected by the phenomenon of group 

 agglutination, since the action of minor 

 agglutinins can be always easily elim- 

 inated by sufficient dilution. Thus if 

 we possess a typhoid-immune serum 

 which agglutinates the typhoid bacillus 

 in dilutions of 1 to 10,000, the para- 

 typhoid bacillus 1 to 1,000 and the 

 colon bacillus 1 to 100 (as in the fig- 

 ure), we may still utilize this serum for 

 the identification of suspected typhoid 

 cultures, us, let us say, in the isolation 

 of unknown bacteria from stools or 

 urine, by using potent sera in dilutions 

 as high or higher than 1 to 1,000, be- 

 yond which point the action of minor 

 agglutinins is eliminated. The dia- 

 gram illustrates our meaning in the 

 hypothetical case of a typhoid-immune 

 serum which agglutinates typhoid in 

 dilutions of 1 to 10,000, paratyphoid 

 bacilli 1 to 800, and colon bacilli 1 to 



100. The relation of agglutination to biologic relationship is not a 

 simple problem in that individual strains even of the same species 

 may vary considerably in agglutination by the same serum. Smith 

 and Reagh 41 have studied particularly these conditions as they pre- 

 vail in the colon, hog cholera and allied groups. They found that 

 biologic relationship usually may be concluded from close agglutina- 

 tion affinities, and that minor biologic differences such as colony 

 appearance, etc., do not exclude such affinities. On the other hand, 

 closely related bacteria vegetating on mucous surfaces (different 

 strains of diphtheria, dysentery, and colon bacilli) may vary con- 

 siderably in their agglutinative characteristics, while invasive species 

 show a greater homogeneity among their varieties or races. This 

 brings in another important feature that is, the modification in 



41 Smith and Reagh. "Studies from the Rockefeller Institute," Vol. 1, 

 1904, p. 270. 



BOO 



SOO 



T 



TYPHO/D PARATYPHOID COLON 



DIAGRAMMATIC REPRESENTATION 

 OF GROUP AGGLUTINATION. 



