208 INFECTION AND IMMUNITY. 



and Trenell distinguish three groups of bacteria 

 in regard to their agglutinability by the homolo- 

 gous antiserums. 1 The first group includes easily 

 agglutinable organisms, for the most pathogenic: 

 Typhoid,, dysentery, cholera, plague, glanders, and 

 the colon, psittacosis, pyocyaneus bacilli, and B. 

 enteritidis. They yield agglutinating serums read- 

 ily either as a result of infection or by immuniza- 

 tion. The second group comprises organisms 

 which, during infection or convalescence, do not 

 cause the formation of agglutinins, but may be 

 forced to do so by systematically injecting them 

 into animals. In the third group are included 

 those which even during prolonged immunization 

 rarely cause the formation of agglutinating 

 serums: the Friedlander bacillus. These facts 

 may be taken as an index of the diseases in which 

 we may expect to obtain the agglutination reaction 

 by the serum of the patient. 



variations in The degree of agglutinating power which may 

 genie Power be obtained by immunization varies greatly. Van 

 ns * der Velde speaks of a typhoid serum which in a 

 dilution of one in one million was agglutinating, 

 and Durham had a cholera serum which was ef- 

 fective in a dilution of one in two millions. Such 

 powerful serums are rarely obtained. 



Even two different strains of the same organism 

 may differ in their ability to cause the formation 

 of agglutinins. It is generally said that a typhoid 

 strain, which is agglutinated with difficulty, gives 

 rise to a weak agglutinating serum, while an easily 



I. The homologous organism for a typhoid serum, for ex- 

 ample, is the typhoid bacillus, and vice versa; other organ- 

 isms, or other serums, are heterologous. These are commonly 

 used terms. 



