THE IMMUNITY UNIT. 
The unit for measuring the strength of diphtheria antitoxin estab- 
lished by Ehrlich is a measure of strength, not of quantity. 
It is difficult to define the unit in a brief sentence. A proper 
understanding of it may only be had from a study of the theoretical 
considerations involved. 
The unit may be defined as the neutralizing power possessed by an 
arbitrary quantity of diphtheria antitoxic serum kept under special 
conditions to prevent deterioration in an authorized laboratory. 
From a theoretical viewpoint the unit mav be defined as that quan- 
tity of diphtheria antitoxic serum which will just neutralize 200 mini- 
mal lethal doses of a pure poison. By a “pure" poison is understood 
one containing only toxin, and no toxoid^ toxone^ or other substances 
capable of uniting with the antibodies. 
The test by which the strength of antitoxin contained in a unit is 
measured is a physiological one and depends upon the neutralization 
of the toxine by the antitoxin. This neutralization can only be deter- 
mined by injecting the toxine and antitoxin mixtures into guinea 
pigs, which animals are highly susceptible to the diphtheria bacillus 
and its poisons. 
In order to obtain a fundamental understanding of Ehrlich's immunity 
unit it is necessary to make a study of the nature of the various 
poisonous substances which are present in the diphtheria toxine. and 
then clearly to understand the combining and neutralizing action 
which these poisons exert upon the antitoxin. Both of these subjects 
are treated more fully in the next chapter. 
In all the earlier work on this subject the toxine was used as a basis 
for measuring the strength of the antitoxin, but as the toxine is a 
much more complex substance than the antitoxin, and as it is less 
stable, accurate results were not possible. Ehrlich showed that the 
antitoxin under certain conditions was permanent, both in power of 
chemically combining with and physiologically neutralizing the toxine. 
The unit for measuring the strength of diphtheria antitoxin is an 
arbitrary quantity, just as the units of all systems of weights and 
measures are fundamentally arbitrary quantities, so that the question 
of the quantity of immunity units which a particular diphtheria anti- 
toxic serum contains can only be determined by comparing it with the 
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