156 IMMUNE SERA 



of his investigations was the establishment of the 

 well-known doctrine of phagocytosis, the principle 

 of which is that the wandering cells of the animal 

 organism, the leucocytes, possess the property of 

 taking up, rendering inert, and digesting micro- 

 organisms which they may encounter in the tissues. 

 MetchnikofI believes that susceptibility to or 

 immunity from infection is essentially a matter 

 between the invading bacteria on the one hand 

 and the leucocytes of the tissues on the other. He 

 realizes that the serum constituents play an im- 

 portant role, but this r61e consists in their stimulat- 

 ing the leucocyte to take up the bacteria. 



Thus if a highly virulent organism is injected 

 into a susceptible animal, the leucocytes appear to 

 be repelled, and to be unable to deal with the 

 microbe, which multiplies and causes the death of 

 the animal. If, however, the suitable immune 

 serum is injected into the animal before inoculation, 

 the phagocytes attack and devour the invading 

 micro-organisms. Admitting that the phagocyte 

 plays an important part in certain infections the 

 question must still be considered whether the 

 immune serum has acted on the injected microbes 

 or on the phagocytes. Metchnikoff, we have seen, 

 takes the latter view. 



In 1903 A. E. Wright * called attention to certain 

 substances present in serum which acted on bacteria 



1 Wright and Douglas, Proc. Royal Society, Vol. 72, 1903. 



