478 APPLICATION OF METHODS OF BACTERIOLOGY 



either in a test-tube or at the site of inoculation in the 

 Hving animal body. 



This serum with which we have been experimenting is 

 the so-called "diphtheria antitoxin" or "antidiphtheritic 

 serum." 



For practical purposes, it is obtained from horses, the 

 animals being treated with gradually increasing doses of 

 diphtheria toxin until they are able to withstand enormous 

 multiples of the ordinarily fatal dose. When this point is 

 reached, the protective body — the antitoxin — is present in 

 the blood in such large quantities that the serum may be 

 successfully employed in the treatment of diphtheria in 

 human beings — i. e., as an antidote to the diphtheria toxin 

 that is produced by the growing bacteria in the throat, or 

 elsewhere, and distributed through the body by the cir- 

 culating fluids. 



The Standardization of Diphtheria Antitoxin.^ — The value 

 of diphtheria antitoxin may be determined according to 

 several different standards. Those that are best known 

 have been proposed by Behring and by Ehrlich. 



1. Behring's Method. — He designates as a "normal" poison 

 a toxin of which 0.01 c.c. suffices to kill a guinea-pig weigh- 

 ing 250 grams in four days. Of such a normal diphtheria 

 toxin 1 c.c. will be suflicient to kill 100 guinea-pigs weigh- 

 ing 250 grams each, or 25,000 grams in weight of guinea- 

 pigs. 



The quantity of antitoxin that is required to just protect 

 25,000 grams weight of guinea-pigs from the minimum fatal 

 dose of the toxin is called one immunizing unit. If an 

 immune serum contains in 1 c.c. one immunizing unit, it 

 represents a "normal" antitoxin. 



To determine the strength of an immune serum, 1 c.c. of 



