260 INFECTION AND IMMUNITY 



It is obvious, therefore, that because of the great difficulty in 

 estimating the very slightest evidences of toxic action in guinea-pigs, 

 a more exact method of standardizing the poisons against antitoxin 

 would be to determine how much toxin would be required to neu- 

 tralize one antitoxin unit and still be sufficiently in excess to cause 

 the death of a guinea-pig of 250 grams in four to five days. This 

 would then correspond to the action of one toxin unit, unmixed 

 with antitoxin. A priori it would seem that this value (expressed by 

 Ehrlich as " Limes death" or "L+") must simply be Lo plus one 

 toxin unit. This, however, was found not to be the case. Thus, 

 in the example given, in which T (the toxin unit the quantity of 

 the bouillon killing a guinea-pig of 250 grams in four to five days) 

 was equal to 0.01 c.c., L (the quantity of toxin completely neu- 

 tralizing one antitoxin unit) was found to be 1 c.c. or 100 T. In 

 this same poison, however, L + (the quantity of toxin necessary both 

 to neutralize one antitoxin unit and yet to be sufficiently in excess 

 of neutralization to kill a guinea-pig of 250 grams in four or five 

 days) was not found to be merely L -[-IT; but on actual experi- 

 ment proved to be L -\- 101 T. 



Expressed graphically, the conditions may be stated as follows: 



T= .01 c.c. of the toxin bouillon. 



L + (neutral, of 1 antitox, unit yet killing 1 pig) == 2.01 c.c. of 201 T. 



L (complete neutral, of 1 antitox. unit) = 1. c.c. or 100 T. 



Difference = 1.01 c.c. or 101 T. 



Ehrlich, at first, endeavored to explain this surprising phenomenon 

 on the basis of toxoids. He argued that the toxoids formed by 

 deterioration of toxin might be conceived as possessing three dif- 

 ferent degrees of affinity for antitoxin. If their affinity for anti- 

 toxin were equal to, or more marked than, that of the toxin itself, 

 they could have no influence upon the dose L . If, however, their 

 affinity for antitoxin were weaker than that of toxin, each fresh 

 toxin unit added to the dose LQ would, first uniting with antitoxin, 

 replace a corresponding quantity of these nontoxic substances of 

 weaker affinity, and L + would be reached only after all of these 

 "epitoxoids," as Ehrlich called them, had been replaced, and toxin 

 became free in the mixture. 



