102 AGGLUTINATION. 



The cholera serum acts strictly in accordance with the rules stated above 

 for specific agglutinins, i.e., marked agglutination with homologous bac- 

 teria; very weak, with heterologous. The typhoid serum on the other hand, 

 although in the main it fulfills the same requirements, nevertheless mani- 

 fests some important differences when mixed with heterologous bacteria. 

 It has practically no influence upon the cholera vibrio with which the 

 typhoid bacillus is not at all related; its agglutination of i : 10 can be attained 

 even by a normal serum. The colon bacillus which closely resembles the 

 typhoid, morphologically, but which has very different biochemical proper- 

 ties is more strongly agglutinated, i : 25; while the paratyphoid bacillus, 

 very much like the typhoid bacillus, both morphologically and biologically 

 is agglutinated even in larger dilutions, i : 100. This entire phenomenon, is 

 an expression of the biological relationship of the various bacterial groups 

 and is known as group reactions. 



An understanding of group reactions is to be found in a more 

 . , . . complete conception of specificity. From this source we have 



learned that the difference in antibodies is influenced by the 

 dissimilarity of the injected antigen. For example, the difference between 

 the cholera and typhoid agglutination is caused by that existing in the 

 protoplasmic structure of the respective bacteria. As these bacteria, how- 

 ever, are not constituted of a distinct chemically defined substance, but 

 made up of a mixture of various substances there may be a number 

 amongst them which can act as antigens. If, figuratively speaking, there 

 are five different elements in the body of the typhoid bacillus which can 

 act as agglutinogens, i.e., antigens, these should be able to form according to 

 the law of specificity five different agglutinins. On mixing a typhoid serum 

 with typhoid bacilli, one brings together five distinct antigen antibody com- 

 binations and consequently complete and thorough action results of this 

 union. A biological relationship of bacteria implies the existence of some 

 common protoplasmic constituents. Expressed in the same figurative 

 manner the colon bacillus can be said to have antigen number i in common 

 with the typhoid and paratyphoid bacillus and the paratyphoid may have 

 antigen numbers i and 2 in common with the typhoid bacillus. As a result, 

 the typhoid serum will react with colon bacilli by virtue of their common 



