194 INFECTION AND IMMUNITY. 



a vast number of experiments Ehrlich obtained in- 

 formation which permitted him to estimate that 

 200 "binding units" are represented in that 

 amount of diphtheria toxin (hypo the tically pure) 

 which is exactly neutralized by one antitoxin unit. 

 If the entire amount of antitoxin, i. e., 200/200, 

 is added to the quantity of toxin in question, com- 

 plete neutralization of the latter, of course, occurs. 

 In case the toxin is entirely pure, 199/200 of the 

 antitoxin unit would destroy all but 1/200 of the 

 initial toxicity; and 150/200, or 100/200, or 

 75/200, etc., of the antitoxin when added would 

 permit corresponding degrees of toxicity to be 

 demonstrated through animal inoculations. It 

 was found, however, that neutralization did not 

 take place according to this simple scale. The re- 

 sults were complicated, and Ehrlich has found it 

 convenient to express them graphically in the form 

 The Toxin of a "toxin spectrum" (Figs. 1, 2, 3 and 4). For 

 spectrum, example, let 199/200 of the antitoxin unit be 

 added to the proper amount of the toxin, 198/200 

 to another similar amount, 197/200 to another, 

 etc., down to 150/200. In the last mixture, 50 

 out of the 200 binding units which the toxin pos- 

 sesses are free, and these 50, rather than some 

 other 50, are free because they have less affinity 

 for the antitoxin than the 150 units which were 

 bound. It has been found that those units which 

 first become free have a low degree of toxicity. It 

 was thought that they might have lost their toxo- 

 phorous groups, i. e., that they were toxoids; and 

 because of their weak affinity for antitoxin they 

 were called epitoxoids. It was found, however, that 

 they possessed a rather constant though low degree 

 of toxicity and that the toxic action was characteris- 



