SIDE-CHAIN THEORY. 199 



to be the case. A single injection of tetanus toxin 

 may yield 100,000 times the amount of antitoxin 

 necessary to neutralize the toxin injected. An in- 

 teresting experiment is on record which shows the 

 fallacy of the view just mentioned. An animal, 

 the serum of which was rich in 1 antitoxins, was 

 bled repeatedly until an amount of blood which 

 equalled the total quantity normally present in the 

 animal's body was drawn. Yet the antitoxic 

 power of the new formed blood was practically un- 

 changed. 



Metchnikoff, to explain this "overproduction" 

 of antitoxin, has suggested that the toxin molecules 

 may be taken up by phagocytic cells and broken up 

 into an indefinite number of smaller molecules, 

 each of which then is altered in some obscure man- 

 ner so as to constitute a molecule of antitoxin. 



The views of Ehrlich have found wide accept- 

 ance, and have provided a valuable working hy- 

 pothesis for many investigations. A considera- 

 tion of this subject introduces one at once to the 

 well-known side-chain theory of immunity of 

 Ehrlich. It may be considered briefly at this point, 

 in so far as it involves the origin and nature of 

 antitoxin. Ehrlich considers it fundamental, in 

 regard to the metabolic activity of cells, to assume Receptors. 

 that the cell constituents must enter into chemical 

 combination with food substances in order that the 

 latter may be made available for the use of the 

 cell. It is supposed that cells contain cer- 

 tain atom groups of unknown chemical nature 

 which make possible the binding of food sub- 

 stances. The name of receptor was given to such 

 groups, since substances are received into the cell 

 through them. Inasmuch as the foods and some 



