22 IMMUNE SERA 



which the diphtheria bacillus had grown and pro- 

 duced its " toxin," was supposed to require for 

 its neutralization an amount of antitoxin directly 

 proportional to its toxicity as tested in guinea pigs. 

 Thus, if from one bouillon culture ten fatal doses 

 of "toxin" were required to neutralize a certain 

 quantity of antitoxin, it was believed that ten 

 fatal doses from every culture, without regard to 

 the way in which it had been produced or preserved, 

 would also neutralize the same amount of antitoxin. 

 Upon this belief was founded the Behring-Ehrlich 

 definition of an antitoxin unit. 1 



The results of tests by different experimenters 

 of the same antitoxic serum, but with different diph- 

 theria toxins, proved this opinion to be incorrect. 

 Ehrlich 2 deserves the credit for first clearly per- 

 ceiving and calling attention to this fact. He 

 obtained from various sources twelve toxins and 

 compared their neutralizing value upon antitoxin; 

 these tests gave interesting and important in- 

 formation. The following table gives the results 

 in four of his toxins and well illustrates the point in 

 question : 



1 This unit was " ten times the amount of antitoxic serum 

 necessary to just protect a 250 gramme guinea pig against 

 ten fatal doses of the toxin." 



2 Ehrlich, Die Werthbemessung des Diphtherieheilserums., 

 Klinisches Jahrbuch, 1897. 



