19 
standard serum preserved by Ehrlich in the Klg. Institut fiir Experi- 
mentelle Therapie at Frankfort a. M., Germany. This serum has 
been carefully tested, its antitoxic strength is accurately known, and 
by comparing any other serum with this original one its antitoxic 
value may be determined. 
Ehrlich, however, does not admit that his antitoxic unit is such an 
arbitrary quantit}^, and believes that he can reproduce it should it 
become lost, by a study of the peculiar relations which exist between 
the combining and neutralizing power of toxines and antitoxins; for, 
from a theoretical point of view, Ehrlich “ considers that the immunity 
unit contains just 200 ‘‘combining units.” A combining unit is anala- 
gous to the valence in chemistiy, and Ehrlich has shown b}^ his stud}^ 
of various diphtheria poisons that the antibodies in his immunity unit 
have 200 of these combining affinities. As each combining affinity 
represents one minimal lethal dose, the immunity unit theoretically 
should combine with and neutralize 200 MLD’s of a toxine con- 
taining only toxin. If it were possible to obtain the toxin in pure 
solution, free from toxoids^ toxo7ies^ and other substances which have 
the power of uniting with the antibodies, it would then be compara- 
tively eas}^ to demonstrate the number of minimal lethal doses which 
the immunity unit is able to neutralize. As such pure poisons are 
practicall}" never obtained, the immunity unit in actual practice is 
found to neutralize many times less than 200 minimal lethal doses of 
the toxine (from 16 to 136), the remaining combining affinities being 
satisfied by such allied substances as toxoid.^ toxoiie.^ etc., which have 
the same chemical affinity for the antitoxin, but diminished poisonous 
properties. 
In order the better to understand Ehrlich’s immunity unit it will be 
necessary to give a brief review of its development. In all the earlier 
systems of measuring the potency of diphtheria antitoxic serum the 
diphtheria culture or its toxine was the basis of measurement. Behring 
and Roux both first used living cultures. Behring substituted the use 
of the soluble diphtheria poison for living cultures as early as 1893, as 
he found it was impossible to establish any system of definite dosage 
with live organisms. 
The method at first used by Roux for determining the strength of 
antitoxic serum was based upon the ratio between the quantity of the 
serum necessary to protect a guinea pig and the weight of that guinea 
pig. His method was carried out as follows: A certain quantity of 
« Ehrlich: Die wertbemessung des diphtherieheilserums mid deren theoretische 
grundlagen. Klin, jahrb., Jena, v. 6 (2), 1897, pp. 299-326. 
Ehrlich: ETeber die constitution des diphtheriegiftes. Dent. med. woch., Leipzig, 
Y. 24 (38), Sept. 22, 1898, pp. 597-600. 
Ehrlich: Ueber die giftcomponenten des diphtherietoxins. Berl. klin. woch., 
1903, nos. 35-37, p. 37. 
