THE CHEMICAL MECHANISMS OF DEFENCE 1157 



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 toxo- 

 phore 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 ' toxoidsJ They have the same combining power for antitoxins 

 as is possessed by the ordinary toxins, but are either without physio- 

 logical 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 

 which the latter endeavours to adapt itself to by the production of 

 other side-chains of the same character. It may be regarded as a 

 general rule in living tissues that a reaction tends to be an over- 

 reaction, so that the compensation by the cell should more than make 

 good the defect produced by the attachment of the toxin. We thus 

 get, not one, but a number of side-chains produced of the same character 

 as that occupied by the toxin molecule, and therefore able also to act 

 as receptors for the haptophore group of the toxin. These new receptor 

 side-chains, being produced in excess, are supposed by Ehrlich to be 

 thrown off from the cell and to circulate in the body-fluids (Fig. 484, 4). 

 A number of protoplasmic fragments are thus set free which have a 

 specific power of uniting with the toxin, and it is this excess of side- 

 chains thrown off from the cell which represents the antitoxin molecules 

 found circulating in the blood after the injection of toxins. It will 

 be noted that this theory, though chemical in form, is really purely 

 biological. It does not explain the phenomena by reference to the 

 known laws of chemistry, but is a manner of viewing the biological 

 phenomena which facilitates their description and discussion and 



