IMMUNITY AND SUSCEPTIBILITY. 125 



a variable, not a fixed quantity, and while the toxin en- 

 durance of the animal is kept up without variation, the 

 antitoxin may gradually diminish. This I have seen 

 many times illustrated in horses producing diphtheria 

 antitoxin, an excellent illustration being afforded by one 

 particular horse that furnished at one time a serum con- 

 taining 1400 units to each cubic centimeter of serum. 

 The immunity was maintained by cautious toxin injec- 

 tions for a long period subsequently, and the endurance 

 of the horse remained unchanged for months, but the 

 antitoxicity of its blood gradually declined, until from 

 1400 units it fell to 100 units. It was not worth while to 

 keep the horse longer, and it was turned out to pasture, 

 and later used to work about the farm. 



This rather sudden appearance of the antitoxin and its 

 decline during the immunity of the animal prepare us 

 for the information that the animal's immunity does not 

 depend upon the antitoxin, but upon some other condi- 

 tion. The probable proof of this is seen in the pecu- 

 liar condition of hypersensitivity to which Behring and 

 Wladimiroff called attention, and of which mention has 

 been made. In these cases it makes no matter how 

 much antitoxic strength is contained in the blood, the 

 animal is just as sensitive to the toxin as if it had none, 

 and as if it had not been immunized. Moreover, the 

 hypersensitivity is not a cumulative action of the toxin 

 that outweighs the antitoxin, as will be readily shown 

 by a simple calculation. A horse weighing 1300 pounds, 

 possessing about 100 pounds of blood, of which about 

 one-third, or 30 pounds, is serum, has been immunized 

 to diphtheria toxin according to the method described in 

 the chapter upon Diphtheria (q. z>.); and while the serum 

 contains 500 immunizing units of antitoxin in each cubic 

 centimeter of blood serum, the horse falls into the hyper- 

 sensitive condition and dies. What relation exists between 

 the antitoxin in its blood and the toxin that produced 

 death ? 



It is certain that there is none, and that the antitoxin 



