368 COLLECTED STUDIES IN IMMUNITY. 



standard unit of antitoxin. The figures thus obtained, as was to be 

 expected, varied greatly: in one case the antitoxin unit neutralized 

 0.25 cc. toxic bouillon, in another case 1.5 cc. This is not in the least 

 surprising, for it is well known that the amount of poison given off by 

 the bacteria to the medium depends on the strain of the bacilli, on 

 the preparation of the bouillon, etc., so that strong poisons and weak 

 poisons arise. But, assuming that the toxin molecule follows chemical 

 laws in its union with antitoxin, it was to be expected that in the 

 different poisons the amounts neutralized by 1 I. E. (ImmunEinheit= 

 Immune Unit), and designated as LQ, would contain equal amounts 

 of true poison, or in other words that the various poisons which differ 

 in their L doses represent nothing more than more or less concentrated 

 solutions of the same toxic substance. The amount of poison con- 

 tained in a solution is measured in poison units, i.e., that amount of 

 toxic bouillon which just suffices to kill a guinea-pig weighing 250 grm. 

 in four days. Thus if in a certain poison A we find the amount neu- 

 tralized by 1 antitoxic unit, i.e., the L dose, to be 1 cc., and if we 

 further find that 0.01 cc, of the same poison suffices to kill a guinea-pig, 

 we say that in this poison the L dose represents 100 poison units. 

 In accordance with the law of equivalent proportions we should have 

 expected that the L dose of the various poisons would contain the 

 same number of poison units. As a matter of fact, however, the result 

 was quite the reverse, for we found that the number of poison units 

 contained in L varied from a minimum of 10 units to a maximum 

 of 150. According to the view held at that time that the antitoxin 

 was bound only by the toxin, this wide divergence from the laws of 

 equivalence could not help but cause the assumption that the 

 relations existing between these two opposing substances were other 

 than purely chemical ones. 



Finally by employing a method of study which has proved of 

 considerable value in scientific investigations, namely, the genetic 

 method, I succeeded in getting some light on this subject. Follow- 

 ing this I subjected one and the same toxic bouillon to comparative 

 tests at different times. I may be permitted to demonstrate this by 

 means of a simple schematic example. In a freshly made poison 

 we find that the quantity which is neutralized by 1 I. E., in other 

 words the L dose, amounts to 1 cc., and that this contains 100 poison 

 units. If the same poison is examined at the end of about six months, 

 it is found that the L dose is the same, but that this contains only 

 50, i.e., half the number of toxic doses. That is to say, the toxic 



