392 TYPHOID FEVER 



in such a high dilution as to make the differentiation of such a 

 reaction from one indicative of infection difficult. A positive 

 diagnosis cannot be founded on an agglutination with a con- 

 centration of serum higher than 1-100 ; some observers, in fact, 

 would demand a result with a still lower concentration before 

 diagnosing the presence of this variety of dysentery. 



The chief difficulty in interpreting observations of agglutin- 

 ation reactions is met with when in infected or ino.culated 

 individuals the serum agglutinates more than one member of 

 the group. From this point of view the pathogenic members 

 of the coli-typhoid group fall into two sub-groups, one of which 

 contains the typhoid bacillus, the paratyphoid bacilli, and the 

 bacillus of Gaertner, and the other, the two types of dysentery 

 bacilli ; a serum which frankly agglutinates a member of one 

 sub-group usually has little or no effect on the members of the 

 other sub-group. The occurrence of such cross-agglutination is 

 attributed to the fact that the causal organism in an infection 

 not only stimulates the production of agglutinins towards itself 

 (primary or homologous agglutinins), but also of agglutinins 

 acting on kindred species (secondary or heterologous agglutinins). 

 The primary agglutinins are usually formed in greater amount 

 than the secondary, and thus the organism which is agglutinated 

 in the highest dilution may usually be looked on as the causal 

 bacterium. A serum may, however, contain primary agglutinins 

 to more than one organism. This may arise where more than 

 one infection exists at the same time in one individual, or 

 where primary agglutinins originating from a previous infection 

 persist in the body, or — what is at present the most frequent 

 source of confusion — where such persistence is the relic of a 

 previous preventive inoculation. In order to surmount the 

 difficulties arising from such complications some observers have 

 used the method of making frequent — it may be daily — 

 estimations during an illness of the highest dilutions in which 

 the serum will agglutinate each of the organisms which may be 

 suspected to be. the causal agent. This method has been 

 specially elaborated by Dreyer, Ainley Walker, and Gibson, who 

 hold that the study of the curves of the agglutinin content of 

 the serum gives valuable information. Thus, a regular and 

 marked rise in the curve of one of the typhoid-paratyphoid 

 sub-group, to a maximum between the 16th and 24th day 

 (especially between the 18th and 20th), with a gradual fall there- 

 after indicates an infection with that bacillus ; if in such a case 

 primary agglutinins are present towards other members of the 

 sub-group (due, it may be, to a previous vaccination), the curves 



