264 STUDIES IN IMMUNITY. 



differ according to the experimental conditions, and contain with a 

 given dose of one substance variable amounts of the other? 



Before seeking an answer we must recall certain experimental 

 data. In the first place it has been shown that if an amount A of 

 antitoxin is necessary completely to neutralize an amount T of toxin, 

 that 2A, 3A or nA is necessary to neutralize 2T, 3T or nT respec- 

 tively. If we regard the antitoxin as acting directly on the toxin 

 and combining with it, there is nothing surprising in this fact. It 

 would, moreover, seem, a priori, evident. This fact, however, 

 affords no definite information as to whether antitoxin and toxin 

 unite in definite and constant proportions.* 



There is a second very important fact mentioned by Ehrlich. 

 Having determined very exactly the minimal lethal dose of toxin, 

 let us suppose that it is necessary to add 100 fatal doses of this toxin 

 to a quantity A of antitoxin to produce a neutral mixture. We 

 admit that the dose A of antitoxin is just sufficient to neutralize the 

 toxin or, in other words, to produce a harmless mixture containing 

 no excess of antitoxin. Let us now prepare a mixture containing 

 also a dose A of antitoxin, but 101 minimal lethal doses of toxin. 

 We might suppose that this mixture would kill a test animal, since 

 it contains an excess of toxin equal to one fatal dose. This is not 

 what happens, however; the animal shows only slight symptoms. 



Mixtures may be prepared containing a considerably increased 

 amount of toxin, even as much as 200 lethal doses, to the same dose 

 A of antitoxin, without proving fatal for animals within the usual 

 time limit. The injection of such mixtures produces slight edemas 

 if the excess of toxin is slight, and more serious ones if the excess is 

 considerable. 



This is " Ehrlich 's phenomenon." It evidently offers a notable 

 objection to explanation by the law of fixed proportions. The 

 simplest explanation of the phenomenon is evidently the one we 

 have already mentioned, namely, that the antagonistic substances 



* It would seem unnecessary to insist on this fact, if it were not that a bac- 

 teriologist has recently asserted, in an analogous manner, that the agglutinin unites 

 in definite proportions with the agglutinable substance of bacteria, basing the 

 assertion on the obvious fact that, if a dose A of serum is necessary to agglutinate 

 a dose B of emulsion, that 2A is necessary to produce the same effect on 2B. If we 

 were to reason in this way, we should assert that a paint unites in definite propor- 

 tions with the surface of a wall, since, if a quantity A of paint is needed to paint 

 10 square meters, 2 A is necessary to paint 20 square meters. 



