ANTITOXINS 9 



may combine with these side chains it is necessary 

 that certain very definite relations exist between 

 the combining group of the substance and that 

 of the side chain. Using the well-known simile of 

 Emil Fischer, the relation must be like that of lock 

 and key, i.e., the two groups must fit accurately. 

 Hence not every substance will fit all the side 

 chains of an organism. It will combine only with 

 those for which it possesses a fitting group. 



Receptors Weigert's Overproduction Theory. 

 This doctrine of the chemistry of the organism's 

 metabolism Ehrlich applied to the action of toxins 

 and antitoxins. ' The toxin," he said, " can act 

 only when its haptophore group happens to fit to 

 one of the side chains," or receptors, as he now pre- 

 fers to call them. As a result of this combination, 

 the toxophore group is able to act on the cell and 

 injure it. If we take as an example tetanus, in 

 which all the symptoms are due to the central ner- 

 vous system, the side-chain theory assumes that 

 the haptophore group of the tetanus poison fits 

 exactly and is combined with the side chain or 

 receptors of the central nervous system. Other 

 experiments, which we will not reproduce here, 

 have shown us unquestionably that the action of 

 the antitoxins depends on the fact that this com- 

 bines with the haptophore group of the poison and 

 so satisfies the latter 's affinity. Ehrlich, therefore, 

 concluded that the antitoxin is nothing else than 



