600 Introduction to the Study of Science 



An antitoxin possesses far greater capacity for uniting with 

 bacterial toxin than do the side chains that remain attached 

 to the cells. The attached side chains retain largely their 

 nutritive function, and only incidentally combine with toxic 

 substances. Thus the antitoxin acts as a prophylactic, pre- 

 venting the free poisons from reaching and uniting with the 

 cells. It does this as long as it is sufficient in amount and 

 potency, and free toxin is produced in the body. But in cer- 

 tain kinds of infectious disease the toxins unite with cells before 

 these can produce the necessary antitoxins. This union of 

 toxin and cell varies in stability. In tetanus, for example, 

 the poisons unite most readily and firmly with the cells, and 

 especially with nerve cells, which form very little if any of the 

 antitoxin. This union is exceedingly difficult, if not impossible, 

 for an antitoxin to break down. Tetanus antitoxin prepared 

 artificially and injected unites freely with and neutralizes the 

 free toxin in the system. If it is injected directly after infec- 

 tion and in sufficient amounts, it acts as a prophylactic, pre- 

 venting the development of the disease. But when the disease 

 is well developed, that is, when the toxin has united with a 

 great deal of the cellular tissue, the antitoxin may only rarely 

 effect a cure, for it fails to break down the chemical union of 

 cell and toxin. 



How an antitoxin may bring about a cure is made quite clear 

 by the chemical side chain theory. In a disease like diphtheria, 

 where the union of cell and toxin is more easily broken down 

 than in the case of tetanus, an antitoxin may be curative as 

 well as prophylactic. In diphtheria the bacterial poison rap- 

 idly combines with the cells before sufficient antitoxin can be 

 formed to guard them. But as soon as the antitoxin is formed 

 in sufficient amounts by the cells responding to toxic stimula- 

 tion, or as soon as it is injected, it begins not only to absorb 

 and neutralize the free toxins in the system, but also to break 

 down the union of cell and toxin. Thus antitoxin in diphtheria 

 performs a twofold service; it is prophylactic in neutralizing 



