THEORIES OF IMMUNITY 



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mechanism of the living body which is susceptible of being poisoned 

 becomes the protective agent if it circulates in the blood stream. It 

 is obvious that the cast-off side-chains constitute antitoxin. The 

 body as a whole is qualitatively the same after as before these side- 

 chains are formed in excess of the normal cellular needs; the difference 

 is a quantitative one. An animal is naturally immune, according to 



FIG. 7. Side-chains, third order (bacteriolysins, hemolysins and cytolysins). 1, 

 side-chain attached to cell; c, haptophore group; b, complementophile group; 2, side- 

 chain to which is attached a bacterial cell (6) and complement (5) ; 3, a cast-off side- chain 

 of the third order; amboceptor; 4, a cast-off side-chain to which are attached a bacterial 

 cell (6) and complement (5) illustrating lysis; 5, complement. 



this theory, if the cells of the body do not unite with toxin, that is, 

 if they do not contain side-chains which fit the toxin " as a key fits a 

 lock," to use Emil Fischer's analogy. Toxin may circulate in the 

 blood stream of such animals, but it does not unite with the cells. 



Side-chains of the First Order. From the standpoint of the side- 

 chain theory, the toxin molecules consist of two groups a combining 



