VARIATIONS AND VARIETIES . 203 



caused by the bacterial species under consideration, we may 

 find it to be curative or preventive for all Type A infections and 

 not at all so or only lowly so for Type B infections, and 

 vice versa. In other words those specific components of the 

 members of Type A which are revealed by the agglutination 

 test may indicate the possession by Type A organisms of com- 

 ponents that call forth the elaboration by the tissues of the 

 immunized animal of bodies that neutralize only the poisons 

 of the bacteria of Type A that is, of the particular type 

 from which the animal was rendered immune. 



This is a point of fundamental value in connection with 

 the use of antisera for the cure of infections. For the best 

 results such antisera must be homologously related to the 

 "type" organisms concerned in the infection for which it 

 is to be employed. 



As the result of all this, bacteriologists are today concern- 

 ing themselves more with groups and group reactions than 

 with individual species and their peculiarities; that is to say 

 the "typing" of bacteria has become one of the routine 

 operations in bacteriological laboratories, in consequence we 

 speak of the "colon type," the "dysenteric type," the 

 " pneumococcus type," etc., meaning that the particular 

 organism or culture with which we are dealing is a species 

 belonging to one or the other "type" as determined by its 

 agglutinability, and it may or may not conform to all the 

 other reactions by which the so-called typical species is 

 identified. 



Working from such a standpoint we now know that 

 many of the important disease-producing organisms lend 

 themselves to such grouping, and by the adoption of this 

 plan of work it has been possible to make advances and to 

 interpret phenomena otherwise impossible. 



