ANTITOXINS AND RELATED ANTIBODIES 



141 



of the standard antitoxin. Toxin is used which has been pre- 

 served until the first rapid transformation of toxin to toxoid has 

 ceased, and it has, therefore, become relatively stable. A series 

 of syringe tubes is prepared (Fig. 70), each one containing one 

 standard immunity unit of antitoxin, and to these are added 

 varying amounts of the toxin to be tested; each is then injected 

 into a 250-gm. guinea-pig. The amount of antitoxin will be more 

 than sufficient to neutralize the toxin in some cases, and the 

 animals injected will show no ill effects; in other cases the toxin 

 will be in excess, and the animals will die. The amount of toxin 

 which, when thus mixed with 1 U. I., will kill a 250-gm. guinea- 

 pig in just four days is called the L+ dose. The amount of the 



Fig. 70. Battery of syringe tubas for testing the potency of toxin and anti- 

 toxin (Madsen). 



toxin solution just necessary to neutralize an immunity unit, as 

 evidenced by the almost complete lack of tissue reaction at the 

 site of injection, is called the LO (limit zero) dose. In a solution 

 containing toxins only (no toxoids) the difference between the L + 

 and the LO dose would be 1 M. L. D. of the toxin, but such toxoid- 

 free solutions cannot be obtained; hence the difference between 

 the two is greater. The only reason that the LO dose is determined 

 in practice is that a toxin solution, in which the L-f and LO doses 

 differ widely, is not suitable for carrying out the test and should 

 be discarded. When the L4- dose of the toxin has been satis- 

 factorily determined, it may then be used to determine the potency 



