THE THEORY OF TOXIC ACTION. 179 



still be capable of uniting with exactly the same proportion of 

 antitoxin molecules. From these and other circumstances, 

 Ehrlich has advanced the view that the toxin molecule has a 

 very complicated structure, and contains two atom-groups. One 

 of these, the haptophorous {airretv, to bind to), is that by which 

 combination takes place with the antitoxin molecule and also 

 with presumably corresponding molecules naturally existing in 

 the tissues. The other atom-group he calls the toxophorous, and 

 it is to this that the toxic effects are due. This atom-group is 

 bound to the cell elements, e.g. the nerve cells in tetanus, by the 

 haptophorous group. Ehrlich holds that the toxophorous group 

 is the more complicated and also the less stable. It is known 

 that, for instance, a diphtheria toxin obtained by the filtration of 

 a bouillon culture loses its toxicity when subjected to such 

 physical agencies as light and heat, and to certain chemical 

 substances. Ehrlich explains this on the theory that the toxo- 

 phorous group undergoes disintegration. And if we suppose 

 that the haptophorous group remains unaffected, we can then 

 understand how a toxin may have its toxicity diminished and 

 still require the same proportion of antitoxin molecules for its 

 neutralisation. To the bodies whose toxophorous atom-groups 

 have become degenerated, Ehrlich gives the name toxoids, and 

 more recently he has called them toxones. He states that he 

 has found evidence that similar bodies may be directly formed 

 by the diphtheria bacillus and not as the result of subsequent 

 degeneration. Such observations are of importance, not only 

 as throwing light on the constitution of the toxin molecule, but 

 also as affording an explanation of how altered toxins (toxoids) 

 can act as immunising agents by stimulating antitoxin formation. 

 The theory may also afford an explanation of what has been 

 suspected, namely, that in some instances toxins derived from 

 different sources may be related to one another. For example, 

 Ehrlich has pointed out that ricin produces in a susceptible 

 animal body an antitoxin which corresponds almost completely 

 with that produced by another vegetable poison, robin {vide supra), 

 though ricin and robin are certainly different. This may be due 

 to the fact that robin is a toxoid of ricin, i.e. their haptophorous 

 groups correspond, while their toxophorous differ. The evidence 

 on which Ehrlich's deductions are based is of a very weighty char- 

 acter, and will be again referred to in the chapter on Immunity. 



