i8 IMMUNE SERA 



are able to combine with the greatest variety of 

 foreign substances and convert these into nourish- 

 ment suitable to the requirements of the active 

 central body. They are comparable to the pseudo- 

 podia of the lower animals, which engulf food par- 

 ticles and assimilate the same for the immediate 

 use of the organism. In order that any substance 

 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 



