IMMUNIZING RESPONSE. 125 



loses in virulence, if we may judge from the low 

 infectivity usually accredited to tertiary lesions, 

 in spite of the fact that they contain the spiro- 

 chetes. This mutual adaptation, however, does not 

 mean that the host escapes injury as a consequence 

 of the infection. Similar conditions probably pre- 

 vail in piroplasmosis and trypanosomiasis, or even 

 in relapsing fever, as suggested previously. 



What has come to be known as the hypothesis of Hypothesis 

 Welch may be mentioned in this connection. It 

 may be put in the form of the following question : 

 If bacterial toxins and the constituents of bacterial 

 cells so act on the tissue cells that the latter pro- 

 duce bodies (antibodies) which are inimical to the 

 bacteria, why may not the body fluids in turn so 

 act on the bacteria that the latter produce bodies 

 (antibodies) which are inimical to the tissue cells? 

 "Looked at from the point of view of the bac- 

 terium, as well as from that of the animal host, 

 according to the hypothesis advanced, the struggle 

 between the bacteria and the body cells in infec- 

 tions may be conceived as an immunizing contest 

 in which each participant is stimulated by its 

 opponent to the production of cytotoxins hostile 

 to the other, and thereby endeavors to make itself 

 immune against its antagonist" (Welch). 



That bacteria may acquire increased resistance JJJJJons'e'b 

 to the destructive agencies of the host was referred J5? c JinJ gmB 

 to above; but the hypothesis of Welch means a 

 great deal more than the immunization of the bac- 

 teria against the defensive powers of the animal 

 body. Not only may a bacterium during an infec- 

 tion become more resistant to the bactericidal 

 action of the body by producing antibodies to those 

 bactericidal agencies, or by its ability to absorb 



