PATHOGENIC BACTERTA. 285 



unit was taken to be 10 times that amount of antitoxic serum which just neu- 

 tralized 10 fatal doses of toxin for guinea-pigs weighing 250 grams. In other 

 words, the exact amount of a certain toxin required to kill a guinea-pig weigh- 

 ing 250 grams in four days having been determined by inoculating a number 

 of guinea-pigs, ten times this amount was put into each of a number of test- 

 tubes, and the antitoxin to be tested was added, a slightly different amount to 

 each tube of toxin. The contents of each tube was then injected into a separate 

 guinea-pig. 



If any of the animals survived, the amounts of antitoxin in the tubes with 

 which they had been inoculated having been noted, the smallest of these amounts 

 — i. e., the smallest amount found necessary to neutralize the toxin — was re- 

 garded as one-tenth of an antitoxic unit. It was naturally assumed that 10 

 times this amount of antitoxin would neutralize 100 fatal doses. This, how- 

 ever, was found not to be the case (see Immunity, page 187). So the revised 

 standard now employed in Germany, France, America and other countries is 

 the unit recommended by Ehrlich. This consists of comparing the antitoxin 

 to be standardized with antitoxin specially prepared by Ehrlich for the pur- 

 pose. This antitoxin of Ehrlich is supplied to the various public and private 

 institutions where antitoxin is prepared, and is carefully standardized against 

 very fresh toxin, which therefore contains little toxoid. 



The Ehrlich standard antitoxin is really used in the first place to determine 

 the strength of a given toxin, which in turn is used to determine the value of 

 antitoxin to be standardized. The actual method is to mix varying amounts 

 of the toxin to be tested each with one unit of the standard antitoxin, and that 

 mixture which just suffices as proved by experiment to kill a 250-gram guinea- 

 pig in three or four days is designated L-(- (see Immunity, page 188); the 

 mixture which is just neutral is called LO. That amount of antitoxin which 

 just neutralizes L-1- contains one antitoxic unit according to this method of 

 standardizing. 



The injection of guinea-pigs with antitoxin serves the double purpose of 

 determining the potency of the antitoxin and also of determining the presence 

 or absence of pathogenic substances, such as tetanus toxin. 



It has been found possible to prepare antitoxin of a high 

 degree of concentration, so that 500 to 1500 units may be 

 contained in a quantity of serum which it is practicable to 

 give at a single hypodermic injection. The large volume of 

 statistics that have been collected from hospitals and from 

 physicians in private practice indicates that the use of diphtheria 

 antitoxin has effected a very great reduction in the mortality 

 from diphtheria. 



