IMMUNITY AND SUSCEPTIBILITY 703 



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 hapto- 

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

 with the cells. Toxin and antitoxin will always combine, if the 

 opportunity presents itself, before toxin and body cells will enter into 

 chemical union. Furthermore, in certain instances, such as in diph- 

 theria, when the toxin has been partially bound by the body cells and 

 antitoxin is produced in sufficient quantities or is injected, the toxin- 

 cell chemical union will be broken up and the toxin and antitoxin will 

 combine. Obviously, antitoxins of this kind are very valuable in 

 effecting a cure in certain infections. In the above-mentioned cases, 

 the union between the toxin and the cell is comparatively unstable but 

 this is not true in every case, as for example, in tetanus or lockjaw. 

 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 up the chemical 

 combination by the addition of antitoxin. It requires exceedingly 

 large doses and these rarely act efficiently. The union between toxin 

 and body cells in this instance is very stable. We have here an ex- 

 planation why tetanus antitoxin is of so little use for therapeutic 

 purposes. It is, however, of use as a prophylactic when free toxin is 

 being produced in the body. Diphtheria 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, and chemicals, affect it and reduce the potency. 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 ex- 

 perimental study to have a unit or standard of measurement. 



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

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

 called antitoxic units it contains. 



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

 antitoxin it is noted that adding fractional amounts of the antitoxin 



