THE CHEMICAL MECHANISMS OF DEFENCE 1035 



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 

 made 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 



FIG. 483. Schematic representation of formation of antitoxin as side-chains of pro- 

 toplasmic molecule. The black bodies are the toxin molecules which fit by 

 their haptophore end on to the side-chains of the cell. (EHRLICH.) 



from the cell and to circulate in the body- fluids (Fig. 483, 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 enables us to classify the very complex phenomena of 

 immunity in a more or less imperfect fashion. 



The property of giving rise to anti-bodies on injection into an animal 



