INFECTION, IMMUNITY, AND RECOVERY. 125 



small amount of specific serum, which robs the tissues 

 of nothing, produces the same immunity. Again, the 

 injection into the body of a sufficient number of patho- 

 genic bacteria gives rise to an infection in all cases. 



The theory of Metchnikoff, that the leucocytes or 

 wandering cells of the body, after an infection with a 

 certain variety of bacteria, become influenced in some 

 way, so that they attack especially that form of infection 

 again and destroy the bacteria (phagocytosis), can no 

 longer be considered as more than a very partial ex- 

 planation, and can only be accepted by assuming as 

 proven a number of hypotheses. 



The retention theory of Wernich and Chaveau, some- 

 what modified, has much to support it. The blood- 

 serum of animals recovering from an infection was found 

 to have changed chemically to such an extent as to be 

 capable of being demonstrated experimentally, and 

 these changes were shown to persist for a number 

 of weeks or months or even years. Similarly the serum 

 of immunized animals retains for a long time its immu- 

 nizing substances We are, therefore, compelled to ac- 

 cept the fact, that when an infection is passed through 

 there are more or less protective chemical substances 

 left in the blood, which remain there for a considerable 

 time. Kruse believes that these substances have the 

 power of neutralizing the bacterial poisons which are 

 given off by the bacteria upon their entrance into the 

 body, and of thus robbing them of their deleterious 

 effects on the alexines; the body fluids in this way 

 remaining unsuitable soil for the growth of the bac- 

 teria, the alexiues being bactericidal. If only a small 

 amount of antilysines are present some of the alexines 

 are destroyed and the bacteria are not all killed or 



