424 BACTERIOLOGY. 



the animals being treated with gradually increasing 

 doses of diphtheria toxin until they are able to with- 

 stand 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 quan- 

 tities that the serum may be successfully employed in 

 the treatment of diphtheria in human beings /. e., as 

 an antidote to the diphtheria toxin that is produced by 

 the growing bacteria in the throat, or else where, and 

 distributed through the body by the circulating blood. 



THE STANDARDIZATION OF DIPHTHERIA ANTI- 

 TOXIN. The value of diphtheria antitoxin may be de- 

 termined according to several different standards. Those 

 that are best known have been proposed by Behring and 

 Ehrlich. 



1. Behrincfs Method. He designates as a "normal" 

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

 pig weighing 250 grammes in four days. Of such a 

 normal diphtheria toxin 1 c.c. will be sufficient to kill 

 100 guinea-pigs weighing 250 grammes each, or 25,000 

 grammes in weight of guinea-pigs. 



The quantity of antitoxin that is required to just pro- 

 tect 25,000 grammes weight of guinea-pigs from the 

 minimum fatal dose of the toxin is called one immuni- 

 zing 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 normal toxin is mixed with increasing quantities of 

 the serum, and these mixtures are injected subcutaneously 

 into guinea-pigs ; the quantity of the serum which suf- 

 fices to neutralize that amount of normal toxin i. e., 

 that keeps the animal alive for four days or longer 

 contains one immunizing unit. 



