FACTS AND PROBLEMS OF IMMUNITY 315 



adjustment of the cellular mechanism to the infection, although if we 

 simply followed the theory of the bactericidal action of serum and 

 plasma as being coextensive, and the active protective mechanism, the 

 rabbit should have been protected, while the dog should have suc- 

 cumbed. The difference here probably depends upon adequate phago- 

 cytosis in the dog, while in the rabbit either the mechanism of ingestion 

 is incomplete or the cells fail to cope successfully with their contents. 

 As a matter of fact, we are coming more and more to the conclusion 

 that actual bacteriolytic processes within the circulating plasma are 

 much less important in protection against invading bacteria than was 

 formerly supposed by the humoral school of bacteriologists. 



Even if the evidence so far in our possession warranted the conclu- 

 sion that the bactericidal and bacteriolytic bodies which are present 

 in the sera of various animals are present and active against certain 

 microorganisms in the same manner in their plasma, we should, never- 

 theless, still have a number of microorganisms which are singularly 

 insusceptible to such action of the serum or plasma, even of animals 

 highly immunized against them. The method of resistance against 

 these would have to be explained by a different mechanism, and if this 

 death and destruction are not accomplished in the plasma, then we must 

 look largely to the activities of the leucocytes for their accomplishment. 



Now, not only the serum substances which further leucocytosis have, 

 as we have seen, received much attention of late, but the bodies an- 

 tagonistic to the bacteria which are supposed to be contained in the leu- 

 cocytes have also been extensively investigated. 



Experiments bearing on these questions make it appear extremely 

 probable that bactericidal and bacteriolytic actions depend on two 

 processes; one of these is the bacteriolytic action of the serum and 

 plasma, the other the bactericidal action of substances retained in the 

 leucocytes. As an example of the type supposed to depend solely on 

 the bactericidal substances of the serum or plasma, the mechanism of 

 the natural and artificial immunity of guinea-pigs to typhoid and cholera 

 may be cited, since in these animals no one has as yet succeeded in 

 demonstrating that substances derived from the leucocytes by extrac- 

 tion have any bactericidal action on the organisms of these two diseases. 

 This does not mean, however, that the bactericidal action takes place 

 naturally outside of the leucocytes, for the bacteria loaded with sensi- 

 tizers are probably taken into the leucocyte and there digested. As 

 examples of immunity depending on substances within the leucocytes, 

 the natural and artificial immunity of dogs and cats to anthrax, and the 



