THE CHEMICAL MECHANISMS OF' DEFENCE 1083 



entirely chemical nor entirely physical, but depends for its existence on a 

 co-operation of both chemical and physical factors. 



How are we to account for the production of the antitoxin as a result 

 of the injection of toxins into the body, a production which is proportional 

 to, but far transcending in amount, the toxin injected ? In all the specula- 

 tions on the mode of production and action of antitoxins, an important 

 part has been played by a conception put forward by Ehrlich in 1885 of the 

 nature of the living protoplasmic molecule. According to this conception, 

 which is spoken of as the * side chain theory,' each unit of living matter 

 consists of a centrally placed protein group with a number of side-chains 

 attached to it, on the analogy of the hypothetical configuration of the 

 benzene ring, to each corner of which may be attached an aliphatic chain. 

 To explain the phenomena of nutrition and oxidation, Ehrlich regarded some 

 of these side-chains as corresponding to unoxidised food substances, while 

 others of the side-chains had a strong affinity for oxygen and might be 

 regarded, when fully saturated with this substance, as peroxide in character. 

 Activity in such a unit would be associated with interaction between these 

 two sets of side-chains. As a result the food chain would be converted to 

 carbon dioxide and an affinity left unsaturated until it could take up another 

 food molecule. In the same way the oxygen side-chain, having lost the 

 greater part of its oxygen, would have a strong affinity for this element and 

 would re-saturate itself at the expense of the oxygen brought to it by the 

 blood. Ehrlich regards the toxins as partaking essentially of the same 

 character as the protoplasmic molecule, as being in fact protoplasmic 

 fragments differing only from the protoplasm of the cell in the greater 

 simplicity of arrangement of their side-chains. According to him the central 

 group, or nucleus, of the toxin possesses two side-chains, one of which by its 

 stereomeric configuration is peculiarly adapted to fit on to the organ or cell 

 of the body which the toxin or active body attacks, and is known as the 

 haptophore group; and another side-chain, the toxophore group, which is 

 responsible, when the toxin is once anchored, for the destructive changes 

 wrought by the toxin on the cell of the body. The antitoxins or antilysins 

 are thus supposed to act in virtue of their adaptation to the haptophore 

 group, so as to combine with the toxin or lysin and prevent these from 

 exercising their injurious effects on the body. Ehrlich has shown that, in 

 many toxins, the toxophore can undergo weakening or destruction without 

 any alteration of the haptophore group; such modifications he designates 

 as ' toxoids.' They have the same combining power for antitoxins as is 

 possessed by the ordinary toxins, but are either without physiological effect, 

 or their poisonous characters are only a fraction of that possessed by ordinary 

 toxin. 



The formation of antitoxins is accounted for (or rather described) on this 

 hypothesis in the following manner. When a receptor side-chain of the cell 

 is occupied by becoming attached to the haptophore group of the toxin, 

 this side-chain is, so to speak, shut out from the normal activities of the cell. 

 A defect is thus produced in the cell, to which the latter endeavours to adapt 



