442 ILEMOLYSINS AND ALLIED BODIES 



After this assimilation is complete, the food molecule drops off, 

 as it were, from the side-chains, and so leaves these free to 

 link other similar food molecules to the biogen. 



The molecule of a toxin (T) also possesses, besides the actually 

 toxic portion of it (tox.), a combining side-chain 1 (hap.), and, if it 

 so happen that the mouldings of this fit one of the mouldings on 

 the side-chains of the tissue cells, then will the toxin unite with 

 the cell and produce the symptoms of the disease 2 ((2) in Fig. 21). 

 The most efficient antitoxin would, therefore, be one which covered 

 over the combining moulding of the toxin molecule ; the union of 

 the toxin with a side-chain of a tissue cell would thereby be 

 prevented. This process would best 'be accomplished by the 

 presence in the blood, in a free state (i.e. disconnected from the 

 biogen), of the same side-chains which normally anchor the toxin. 

 Ehrlich supposes that a setting free of side-chains actually takes 

 place when antitoxin is formed ((3) in Fig. 21). Antitoxin, according 

 to him, is -nothing more or less than disrupted tissue side-chains. 

 By the presence of these in the blood, the toxin molecule will have 

 its moulding fitted, in other words, its combining power satisfied, 

 before it gets to the cells, and will no longer be able to poison 

 these ((4) in Fig. 21). Ehrlich compares the mechanism to the 

 attraction of lightning by a mass of iron. If the iron be placed 

 in a house it will increase the risk of the house being damaged 

 by the lightning ; but if placed outside the house will save it, by 

 attracting the lightning elsewhere. The house is the tissue cell, 

 the lightning the toxin, the iron the antitoxin. 



The cell side-chains are called receptors, the combining portion 

 of the toxin molecule the haptophoric group, and the actually 

 toxic portion of the toxin the toxophoric group. An antitoxin acts 

 by fitting on to the haptophoric group of the toxin ; the toxophoric 

 group then becomes functionless because it is not chained to a cell. 



How, now, does the repeated injection of toxin in gradually 

 increasing dosage lead to the liberation in the blood of these same 

 receptors, which, when attached to the cells, attract and fix the 

 toxin to them, and thereby cause cell destruction ? To answer 



1 Under certain conditions, the toxic portion of a toxin molecule may be- 

 come destroyed, but the combining portion remain active. Toxoid (Td. in 

 Fig. 21) is the name given to such a body. 



2 Owing to the ability of the toxines to turn toward some particular cell, 

 Wright calls them "tropines." Thus tetanus toxin, which has an affinity for 

 nerve protoplasm, may be called " neuro-tropic." (Editor's Note.) 



