22 IMMUNE SERA. 



the combining group of the substance and that 

 of the side-chain. To repeat the simile we used 

 above, 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 me- 

 tabolism 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-chains 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 

 the side-chains or receptors which are given off by 

 the cells and thrust into the circulation. The way 

 in which these side-chains or receptors are thrust 



