IMMUNITY AND SUSCEPTIBILITY 723 



Metchnikoff suggested what may be called a phagocytic theory 

 of immunity. According to his ideas and those belonging to his 

 school, the phagocytes, and principally the mononuclear and poly- 

 morphonuclear leucocytes, are concerned in immunity. He explains 

 natural immunity to toxins on the basis of an increased toxin-absorp- 

 tive power on the part of these cells for toxins. He explains natural 

 antibacterial immunity as an increased power of phagocytosis for the 

 invading microorganism by the leucocytes. He conceived that in 

 acquired immunity to toxins these cells develop as the result of an in- 

 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 Metchnikoff. 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. 



