336 A MANUAL OF BACTERIOLOGY 



with different toxin broths yielded different values. This 

 he explained by assuming that diphtheria toxin broth 

 contains not only toxin but also other substances which 

 combine with antitoxin. These substances, though non- 

 toxic, or comparatively so, vary in amount in different 

 toxin broths, and variable results, therefore, may be 

 obtained by the simple method of testing. These sub- 

 stances, having an affinity for antitoxin, are toxoids and 

 toxone. Ehrlich originally described three varieties of 

 toxoids, viz. (1) those having a greater affinity for anti- 

 toxin than toxin itself, protoxoids ; (2) those having the 

 same affinity, syntoxoids ; (3) and those having a less 

 affinity, epitoxoids. 1 He subsequently found that epi- 

 toxoid is a primary secretory product of the diphtheria 

 bacillus, and to mark this change of view named this 

 substance toxone. Toxoids are derivatives of toxin ; 

 they increase in quantity in old toxin broth which has 

 been kept, and which at the same time decreases in 

 toxicity. Toxone also combines with antitoxin, having 

 a less affinity for it than toxin, and while not acutely 

 lethal, induces induration and paralysis. The toxoids 

 are comparatively scanty in a fresh toxin broth and 

 are negligible, but it is otherwise with the toxone, 

 which is always present in appreciable quantity. 

 Owing to the fact that toxone has less affinity for anti- 

 toxin than toxin has, if an exactly neutral mixture of 

 toxin broth and antitoxin be prepared, considerably 

 more than the minimal lethal dose of the toxin broth 

 must be added to render the mixture lethal, because 

 the first portion of the added toxin simply displaces 

 the toxone from its combination with the antitoxin, and 

 is neutralised by the antitoxin so set free. 



Thus, suppose a certain amount of a toxin broth con- 

 tains 90 units of toxin and 10 units of toxone, and to this 



1 See pp. 194-197 for other views on the constitution of diphtheria toxin. 



