THE ORIGIN OF ANTITOXIN THE SIDE-CHAIN THEORY 95 



the cell for which it has a chemical affinity. Now Ehrlich 

 supposes that this affinity for food molecules is situated in certain 

 portions of the molecule of protoplasm in certain groups of atoms 

 which he calls side-chains, receptors, or haptines. [The name " side- 

 chain " was, perhaps, badly chosen. It denotes a possible analogy 

 with complex organic bodies e.g., of the aromatic group which 

 are composed of a central portion (such as the benzene ring) and 

 side-chains, on which many of their reactions depend. The com- 

 parison is not a very close one, and all that is necessary is to 

 regard the molecule of protoplasm as possessing numerous groups 

 of atoms, each of which has an affinity for one of the bodies 

 circulating in the body fluids, and necessary for the life of the 

 molecule in question.] 



On this theory the nutrition of the molecule takes place as 

 follows : A molecule of suitable food substance in the fluid sur- 

 rounding the cell is brought into contact with one of the receptors 

 for which it has a chemical affinity, and the two unite. This 

 is the first step in the process. The food is "anchored " to the 

 cell by means of a receptor, for which it has a specific combining 

 affinity, or which, to use Ehrlich's analogy, fits it like a key fits a 

 lock. The second stage involves a process which we may compare 

 to digestion, by which the food molecule is altered in some pro- 

 found manner, and absorbed, in whole or in part, into the molecule 

 of protoplasm. 



Let us apply this theory to the process by which a cell is 

 poisoned. We will imagine for a moment that the molecule of 

 toxin contains a group of atoms which will unite specifically with 

 the side-chain of one of the body cells. We have already shown 

 that this molecule of toxin contains a haptophore group of mole- 

 cules, in virtue of which it can combine with antitoxin, and a 

 toxophore group, on which its toxic action depends. We can now 

 go farther, and say that the first stage of the intoxication of a cell 

 by means of a true toxin consists in the union of the haptophore 

 group of atoms in the toxin to a receptor of a molecule of proto- 

 plasm, this receptor being one which "fits it like a key fits a 

 lock." Each molecule of protoplasm has innumerable receptors, 

 of which only a certain number are suitable for this toxin. This 

 is the first step. The toxin molecule is now " anchored " to the 

 living cell, and in the second stage the toxophore radicle of the 

 toxin comes into play. We may regard this toxophore group as 

 exerting an enzyme -like action on the protoplasm through the 



