STANDARDISATION OF ANTITOXIN 281 



of toxin, and he termed ten times the amount of antitoxin 

 which protects a guinea-pig against the ten minimal lethal 

 doses a unit (the Behring unit, which therefore = 100 

 minimal lethal doses of toxin), from which the Ehrlich 

 unit, now universally adopted, is derived. Though this 

 method eliminates to a large extent the objections to the 

 Roux method, Ehrlich found that by it the same antitoxin 

 tested 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. There are several varieties of toxoids, viz. 

 (1) those having a greater affinity for antitoxin than toxin 

 itself, protoxoids ; (2) those having the same affinity, 

 syntoxoids ; (3) and those having a less affinity, epitoxoids. 1 

 Toxoids are probably derivatives of toxin ; they increase 

 in quantity in old toxin broth which has been kept, and 

 which at the same time decreases in toxicity. The toxones 

 also combine with antitoxin, having a less affinity for it 

 than toxin, are primary secretory products of the diphtheria 

 bacillus, and while not acutely lethal, induce induration, 

 necrosis, 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 antitoxin 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 acutely toxic, 



1 See pp. 165-168 for other views on the constitution of diphtheria 

 toxin. 



