IMMUNITY AND SUSCEPTIBILITY. 557 



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 

 antitoxin. 



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 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 disproved. 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 

 harmless and differs from both the toxin and the antitoxin in that it is 

 much more stable. 1 



In the beginning all experiments dealing with the union of toxin and 

 antitoxin were performed in the body of an experimental animal (in vivo), 

 but finally Ehrlich showed that they would act and combine equally well 

 in the test tube (in vitro) and could be studied in a much better manner. 



The various toxins are neutralized by their antitoxins with varying 

 rapidity. The concentration of these bodies, the temperature, the char- 

 acter of the medium in which they are placed, and the amount of electro- 

 lytic 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 govern the union of toxin and 

 antitoxin govern other antibodies and their antigens. 



As before stated, toxins have a greater affinity for the free haptophile 

 receptors of cells (free antitoxin) than for those still associated with the 

 cells. Toxin and antitoxin will always combine, if the opportunity pre- 

 sents itself, before toxin and body cells will enter into chemical union. 

 Furthermore, in certain instances, such as in diphtheria, when the toxin 

 has been partially bound by the body cells and antitoxin is produced in 



