BACTERICIDAL ACTION OF NORMAL SERUM 203 



all these effects take place more readily when the absorb- 

 ing bacteria are treated with the corresponding immune- 

 body. The fact that heterologous organisms absorb the 

 bactericidal complement more readily than the haemolytic 

 complement, would point to there being a moiety of com- 

 plement with greater combining affinity for bacteria in 

 general. In fact it seems impossible, in view of the results, 

 to escape the conclusion that this is so ; we might call this 

 moiety ' bacteriophilic complement '. 



If we now consider the fact that bactericidal action for 

 an organism is more readily reduced by the same than by 

 another organism, it appears that two explanations are 

 possible. In the first place, this may be due to the existence 

 of natural immune-bodies. The homologous organisms will 

 absorb these immune-bodies, as well as a certain amount of 

 complement ; hence the bactericidal action readily falls. 

 The treatment with a heterologous organism will lead to 

 the absorption of complement, but it will leave the immune- 

 body for the organism on which the bactericidal action is 

 to be tested ; it will thus reduce the bactericidal action 

 solely by the removal of complement, but a larger amount 

 will be necessary to bring this about. On the other hand, 

 another explanation may hold good ; just as a moiety of 

 the complement content is specially bacteriophilic, so in this 

 moiety certain molecules may have a greater affinity for, and 

 greater toxic action on, one organism as compared with 

 another ; may, for example, have a greater action on V. Metch- 

 nikovi than on B. typhosus. We have not sufficient facts at 

 present to enable us to say which explanation is the correct 

 one, in fact both may be valid in part ; but the fact that 

 there exist differences in affinity of complement in relation 

 to bactericidal and haemolytic action respectively, certainly 

 makes it possible that there may be variations in the affinity 

 of complement molecules for different bacteria. Although the 

 existence of natural immune-bodies may have been proved in 

 certain cases, we are not justified in generalizing from this. 



