322 INFECTION AND IMMUNITY. 



said that antitoxins are efficacious from the fact 

 that they stimulate phagocytosis (absorption) of 

 the toxin, the latter then suffering disintegration 

 in the leucocytes. 



summary. The following statements summarize the phago- 

 cytic theory of immunity as conceived by Metch- 

 nikoff : 



1. Natural immunity to bacteria depends on 

 and is coextensive with phagocytosis and subse- 

 quent digestion of the microbes. Intraleucocytic 

 destruction of the micro-organisms is accomplished 

 by the cytase, possibly aided by intraleucocytic fix- 

 ators. Normal serum is devoid of both fixators 

 and cytase. 



2. Acquired immunity to bacteria depends on 

 the establishment of a heightened phagocytic power 

 as the result of immunization or infection. In 

 diseases like anthrax, in which fixators are not in- 

 creased, this new power is an acquired property 

 of the leucocytes and is independent of any in- 

 fluence on the part of the serum. In diseases like 

 cholera, the new fixators which are formed may 

 render the micro-organisms more susceptible to 

 phagocytosis, but this is probably secondary to 

 increased function on the part of the phagocytes. 

 Both cytase and fixator are produced by the pha- 

 gocytic cells. In acquired active immunity to 

 bacteria the fixators may be free in the perum 

 and plasma, but the cytase is intracellular. In all 

 cases cytase becomes extracellular only as the re- 

 sult of phagolysis. 



3. In passive immunity to bacteria, as when an 

 antibacterial serum is injected for the sake of 

 prophylaxis or cure, the serum is efficacious chiefly 



