558 MICROBIOLOGY OF THE DISEASES OF MAN AND ANIMALS. 



sufficient quantities or is injected, the toxin-cell chemical union will be 

 broken up and the toxin and antitoxin will combine. Obviously, anti- 

 toxins of this kind are very valuable in effecting a cure in certain infections. 

 In the above-mentioned case, the union between the toxin and the cell is 

 comparatively unstable, but this is not true in every case, as for example in 

 tetanus. In this case when once the toxin is combined with the cells of 

 the nervous system and other body cells, it is very difficult to break the union 

 by the addition of antitoxin. It requires exceedingly large doses and these 

 rarely act efficiently. The union between toxin and body cells in this in- 

 stance is very stable. We have here an explanation why tetanus anti- 

 toxin is of so little use for therapeutic purposes. It is, however, of use as a 

 prophylactic when there is free toxin being produced in the body. Diph- 

 theria antitoxin is efficient both as a curative and prophylactic agent for 

 the reasons which have been discussed above. 



Antitoxin, like toxin, is fairly unstable and such agents as heat, light, 

 chemicals, etc., affect it and reduce the antitoxic power. It may, how- 

 ever, be dried and kept for long periods of time in the dark. It is 

 necessary in the commercial preparation of antitoxin and in its experi- 

 mental study to have a unit or standard of measurement. 



Units of Antitoxin. In order to arrive at a standard it is necessary to 

 test accurately a given antitoxin to determine the number of so-called anti- 

 toxic units it contains (p. 482). 



In the accurate study of the neutralization of the toxin by the anti- 

 toxin it is noted that, adding fractional amounts of the antitoxin to 

 the L dose of the toxin and injecting the resulting mixture into test 

 animals (guinea pigs), there is not a corresponding decrease in the 

 toxicity as would be expected. The toxin seems to be made up of 

 various parts. The part just mentioned has a great affinity for the 

 antitoxin, but is not really toxic. Such parts of the toxin molecule are 

 called protoxoids. The protoxoids compose about one-fourth of the 

 amount of toxin necessary to saturate one immunity unit. After one-fourth 

 antitoxin is added the mixtures of toxin-antitoxin become less toxic for the 

 experimental animals down to the point where three-fourths of the amount 

 of toxin necessary to saturate one unit of antitoxin is used. This fraction 

 is the true toxin. Here again the toxicity of the mixture does not decline 

 and it has been demonstrated by Ehrlich and others that this is due to 

 another part of the toxin molecule which has less avidity for the anti- 

 toxin than the toxin itself and the protoxoid. This part of the molecule is 



