12 LECTURES ON IMMUNITY 



toxins. This belief was due to the very different effects 

 which the same quantity of poison exerted on two different 

 individuals of the same species, e.g., diphtheria-poison on 

 guinea-pigs. It was only the practical necessity of having 

 measurements of the force of poisons that led Ehrlich to 

 overcome the great difficulties. 1 



The chief investigations of Ehrlich concern the diphthe- 

 ria-poison and its antitoxin. He wished to determine the 

 lethal dose of this poison for guinea-pigs. For this purpose 

 he injected a great number of guinea-pigs with different 

 doses ; they lay in the neighbourhood of the lethal doses. 

 It may be here mentioned that large guinea-pigs in general 

 endure a greater quantity of poison than do small ones. 

 Ehrlich supposed that the resistency of different individ- 

 uals was proportional to their weight, and on this assump- 

 tion he calculated the dose corresponding to a " normal " 

 guinea-pig of 250 grammes weight. To obtain uniform 

 results he found it necessary to use animals of nearly the 

 same weight, age, and race. In the summer the animals 

 are more resistent and give more uniform results than at 

 other seasons, when they evidently surfer from the changes 

 of temperature and other climatic conditions. 



The lethal dose of diphtheria-toxin was defined by Ehrlich 

 as the quantity which injected subcutaneously into a great 

 number of guinea-pigs causes the death of the larger 

 fraction of the animals in three to four days, the rest of 

 the animals dying about as many before as after this time. 

 According to this method of measurement a great deal of 



J P. Ehrlich: "Wertbestimmung des Diphtheriheilserums," Jena, 1897; 

 "Constitution des Diphtheriegiftes," Deutsche med. Wochenschrift, 1898, No. 

 38; Ehrlich, Kossel, and Wassermann : Deutsche med. Wochenschrift^ 1894. 



