702 MICROBIOLOGY OF DISEASES OF MAN AND DOMESTIC ANIMALS 



is possible to inject the animals with more toxin, thus combining with 

 more cells and finally liberating more antitoxin. 



It should also be noted .that animals of the same species vary in 

 their power to produce antitoxin. The production of the product 

 varies with the age and general condition of the animal and with the 

 duration and the degree of toxicity of the toxin used. On account 

 of this condition it is necessary to establish units or standards for 

 determining the strength of antitoxins. 



As stated in the discussion of natural immunity to toxins, there 

 are some animals Which when' injected with toxins do not possess cells 

 which have receptors open for chemical combination and as a result 

 the toxin remains free in ' the circulation for varying periods of time. 

 For example, as before stated, the frog is immune to tetanus and an 

 injection of toxin will not produce any antitoxin. If tetanus toxin 

 is injected into this animal it will remain in the circulation in the 

 same form as injected and can be with'drawn after a few weeks or a 

 month. 



The Mechanism of the Neutralization of Toxin by Antitoxin. — 

 At one time it was supposed that the antitoxin was but a toxin in a 

 little different form but this has been absolutely disproven. The 

 amount of antitoxin produced is, much greater than the amount of 

 toxin which is injected or produced during an infection. 



The union between toxin and antitoxin is of a definite chemical 

 nature. After these two substances unite the resulting compound is 

 absolutely harmless and differs from both the toxin and the antitoxin 

 in that it is much more stable. 



In the beginning aU experiments dealing with the union of toxin 

 and antitoxin were performed in the body of an experimental animal 

 (m vivo) but finally Ehrlich showed that they would act and combine 

 equally well in the test-tube {in vitro) and could be studied much more 

 easily. 



The various toxins are neutraUzed by their antitoxins with varying 

 rapidity. The concentration of these chemical substances, the tem- 

 perature, the character of the medium in which they are placed, and the 

 amount of electrolytic salts present, are accountable for the differences 

 in length of time of combination. In the main these substances act 

 like most chemicals and some of them show evidences of following the 

 laws of multiple proportions. As a matter of fact the same laws which 



