BACTERIUM DIPHTHERIA 103 



be injected and much blood withdrawn without 

 harming the beast. The horses receive under the 

 skin gradually increasing quantities of this toxic broth 

 until they are able to withstand huge quantities, 

 many times the quantity necessary to kill them if 

 given at first. They are then considered to have 

 some neutralizing substances for this toxin. This 

 neutralizing property is known to be in the blood 

 serum. The horse is then bled, and the serum separ- 

 ated from the red blood cells. It is then tested against 

 the original toxin used for making the injections. This 

 is done by mixing the two in definite parts, allowing 

 the mixture to stand a few minutes, and injecting it into 

 guinea-pigs. By appropriate technique the number of 

 "units" is determined. A "unit" is that quantity of 

 horse serum, or antitoxin, which will neutralize 100 

 times the smallest quantity of toxin necessary to kill a 

 guinea-pig weighing 250 grams (8i ounces). 



The horse serum antitoxin has now a value for clinical 

 purposes, as the quantity to be given can be controlled. 

 Newer methods have permitted the refinement and 

 concentration of this antitoxin, so that there is now 

 less inconvenience in giving it. The dose for treatment 

 varies from 1500 to 5000 units by injection under the 

 skin. In bulk this may be less than a teaspoonful. 

 For immunizing purposes, that is, to protect persons 

 exposed but not yet suffering from the disease, from 

 300 to 1000 units are used. In both cases a repetition 

 of the dose iis frequently demanded, and in case the 

 exudate does not fade, the injections may have to 

 be given several times. The effect is a passive acquired 

 immunity, as it is the addition of a toxin-neutralizing 



