IMMUNITY AND SUSCEPTIBILITY 723 



fection or artificial injection of microorganisms, an increased power of 

 absorption of toxin and the power of producing antitoxin, and that 

 acquired immunity to bacteria-producing endotoxins is due to the in- 

 creased power of the phagocytes to ingest and digest invading micro- 

 organisms. 



We find the best explanation for the phenomena of immunity in 

 both the theories of Ehrlich and Metschnikoff. ' Undoubtedly certain 

 forms or types of immunity are due to definite chemical substances 

 known as antitoxins or bactericidal substances, while other types are 

 due to the activity of the phagocytes. 



