IMMUNITY AND SUSCEPTIBILITY. 575 



are stimulated and produce or open up an excess of these chemical 

 side chains for combination. It is conceived that if enough toxin is 

 assimilated by the cells that these chemical side chains, which are definite 

 chemical substances, will be split off from the original cell compound 

 and escape into the circulation. It is these escaped chemical side 

 chains which constitute the antitoxin, bactericidal substance, agglutin- 

 ins, precipitins, and opsonins. In the case of antitoxins, they possess 

 a maximum affinity for the toxin and will combine with the toxin much 

 more readily than the toxin will combine with the remaining chemical 

 side chains on the original cell compound. In the case of bactericidal 

 substances they will combine with the bacteria and destroy them and 

 liberate in this way the endotoxins which may subsequently combine with 

 antiendotoxin (?) or tissue cells. It may be here stated that inasmuch 

 as no antiendotoxins are ever produced, that the presence of bactericidal 

 substances in a large per cent of instances is a detrimental factor. The 

 production of antiendotoxins by some method or other is extremely de- 

 sirable. Inasmuch as the majority of our diseases are due to bacteria 

 producing endotoxin, such a product would be of immense value in com- 

 bating these infections. The chemical theory of Ehrlich explains many 

 features of the phenomena of immunity. This theory has been the basis 

 of nearly all of the preceding discussions on the various antibodies. 



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 polymorphonu- 

 clear leucocytes, are concerned in immunity. He explains natural 

 immunity to toxins on the basis of an increased absorptive power on 

 the part of these cells for toxins. He ascribes natural antibacterial 

 immunity to an increased power of phagocytosis for the invading micro- 

 organism. He conceived that in acquired immunity to toxins these cells 

 develop, as the result of an infection, an increased power of absorption 

 of toxin and the production of antitoxin, and acquired immunity to 

 bacteria-producing endotoxins is due to the increased power of the 

 phagocytes to ingest and digest invading microorganisms. 



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, bactericidal substances, etc., while other types are due to the 

 activity of the phagocytes. 



