TOXINS AND ANTITOXINS. 193 



The toxin molecule contains not only a hapto- 

 phorous group, through which it unites with anti- 

 toxin in one instance or, in another instance, with 

 tissue cells in the production of disease, but also 

 certain constituents in which the specific activity 

 of the substance resides, and by which it produces 

 changes in tissue cells, on combining with them. 

 This functional or pathogenic activity resides in 

 this so-called toxophorous group of the molecule. 

 Hence the haptophorous and tovophorous groups 

 are the two structural elements of a toxin which 

 may be recognized by biologic experiments. 



It is a peculiarity of toxins that they lose a cer- 

 tain amount of their toxicity in the course of time, 

 although their binding power for antitoxin remains 

 practically unchanged. In the language of the 

 terms which were used above, the toxophorous 

 groups may degenerate or disappear and leave the 

 haptophorous groups intact. Toxins which have 

 undergone this change are called toxoids. 



Further evidence of the existence of toxoids lies 

 in the fact that when used for immunization they 

 cause the formation of antitoxins. This i& possi- 

 ble only when the substance is able to unite with 

 the tissue cells; therefore, the non-toxic toxin or 

 toxoid has retained its haptophorous groups. 



A toxin entirely free from toxoids has never 

 been observed, since even during the few days re- 

 quired for its preparation a certain amount of 

 degeneration occurs. 



Additional information concerning the nature partial 

 of toxin has been gained by experimenting with l^nuSi 

 mixtures of toxin and antitoxin, in which the two study. 

 are present in varying proportions. This is the 

 "partial saturation" method of Ehrlich. Through 



