14 LECTURES ON IMMUNITY 



By the aid of all these different measurements it is 

 possible to determine the probable error of every measure- 

 ment. This was omitted in all previous considerations of 

 this question. Under these circumstances an overestima- 

 tion of the exactitude of the measurements resulted, which 

 often led to conclusions without any solid foundation. 



The lethal dose of the toxin having been determined, the 

 next step was to standardise the solution of antitoxin. 

 For this purpose Ehrlich determined the quantities of the 

 solution in question that were necessary to neutralise a 

 certain number (e.g., 100) of lethal doses of the poison. As 

 experience has taught that the antitoxin is destroyed 

 much more slowly than the toxin, especially if it is con- 

 served with certain precautions, a given preparation of 

 diphtheria-antitoxin is assumed as " standard serum," and 

 the properties of all poisons and antitoxins to be examined 

 are compared with this standard unit. 



Ehrlich in his measurements used the assumption that 

 injections of the same quantities of free poison have the 

 same effect, independently of the quantities of "neutralized 

 poison " present. The correctness of this assumption has 

 recently been subjected to question, to which we will 

 return later. 



The diphtheria-poison can be measured as far as we 

 know only by the aid of experiments on living animals. 

 But there are many other poisons which may be studied 

 outside of the living body, as for instance the hasmolysins, 

 the precipitins, and the agglutinins. Experiments "in 

 vitro " have already long been used in physiological 

 researches, e.g. in digestion, to study processes outside of 

 the living body. Ehrlich adopted this method to study 



