IMMUNITY. 209 



The methods used in the production of antitoxins were intro- 

 duced by Behring, who found that by injecting susceptible 

 animals with increasing amounts of extra-cellular toxin he 

 produced in the blood-serum of the injected animal certain 

 changes which made the serum capable of counteracting the 

 same toxin when injected into other animals. Thus, a sheep 

 treated with increasing doses of diphtheria toxin, beginning 

 with very small amounts, furnishes blood-serum which protects 

 other sheep or guinea-pigs or other susceptible animals from 

 fatal doses of diphtheria toxin. In practice, the bacilli are 

 cultivated in bouillon. The cultures are freed from all living 

 bacilli by nitration. The liquid nitrate contains the toxin. 

 This nitrate is injected into healthy susceptible animals, in 

 increasing doses. Usually the horse is used, since large quan- 

 tities of blood can be drawn from this animal on account of its 

 size, and, moreover, the horse is very susceptible. Insuscep- 

 tible animals cannot be made to yield antitoxin, at least of 

 any appreciable strength. Eventually enormous doses of 

 toxin are given, and the animal acquires a high degree of immu- 

 nity. The blood of the animal is withdrawn, taking care to 

 avoid contamination, and the serum allowed to separate in the 

 refrigerator. The serum of the blood is drawn off and consti- 

 tutes the antitoxin. The use of antitoxin has been eminently 

 successful and revolutionized the treatment of diphtheria; and 

 it has given complete success as a prophylactic in tetanus with 

 an antitoxin prepared by injecting horses with increasing 

 amounts of tetanus toxin. (See the description of the bacteria 

 of these diseases.) 



Ehrlich discovered that the vegetable toxins, abrin and ricin, 

 behave in a manner very similar to soluble bacterial poisons when 

 injected into animals, and that by their injection an immunity 

 for the same poisons may be secured. Ehrlich also found that 

 the milk of animals which had been immunized with increasing 

 doses of abrin and ricin confers immunity upon sucklings. 

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