32 IMMUNE SERA 



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 infor- 

 mation. The table on page 33 gives the results in 

 four of his toxins and well illustrates the point in 

 question. 



It was natural to suppose, as the early investi- 

 gators did, that a just neutral mixture of toxin 

 and antitoxin would require the addition of but 

 one fatal dose of toxin in order to regularly kill 

 the test animal. In the table, however, we see 

 that this difference ranges from six to fifty fatal 

 doses. 



Partial Saturation Method Toxons, Toxoids. 

 Ehrlich obtained considerable additional informa- 



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. Kli- 

 nisches Jahrbuch, 1897. 



