DIPHTHERIA SERUM. 73 



very strong antitoxic qualities. This was done by Salomonsen and Madsen 

 who by experimentation found that the maximum height of the antitoxic 

 curve was reached on the tenth day after each inoculation. For this reason 

 it is wise to choose this day for the removal of the serum. As regards other 

 sera, e.g., tetanus, different periods have been empirically found to be most 

 serviceable. As the antitoxic curve does not remain at a high point for a 

 long time, the injections should be repeated from time to time. For 

 highly immunized horses, monthly injections usually suffice. 



After the serum has been obtained, the important problem which arises 

 is how to keep it sterile. This is accomplished by aseptic precautions at the 

 time of the obtention of the serum and eventually by the addition of pre- 

 servatives such as 1/2 per cent, carbolic. 



This procedure finished, the next step is to estimate the amount of the 

 antitoxin content in the serum. 



According to v. Behring and Boer, the value of the serum should be 

 ascertained in respect to its: 



i. Protective power 



, against infection. 



2. Curative power J 



3. Protective power 1 



~ . > against intoxication. 



4. Curative power J 



v. Behring found that these four properties run parallel with each other 



so that for practical purposes, it suffices to establish only one of these qualities. 



For diphtheria serum it has proved most serviceable to 



Standardiza- estimate the strength of the immunity against intoxication, 



. since one is dealing with a purely antitoxic serum. 

 Diphtheria 



Serum. Behring's original mode of standardization consisted in gradually add- 

 ing doses of serum to the minimal lethal dose of toxin and injecting the 

 mixtures into guinea-pigs, thus determining the smallest amount of serum capable of 

 preventing death of the animal. It was soon found, however, that this method gave 

 too inconstant results because the individual minimal lethal dose was too variable. 



Ehrlich, therefore, modified the process by using ten times the minimum lethal dose. 

 This amount of toxin, mixed with decreasing amounts of serum and made up to 4 c.c. 

 with physiological salt solution was injected subcutaneously into a guinea-pig. The 

 smallest amount of serum which saved it from being killed on the fourth to fifth day was 

 thus estimated. 



The method of standardization used at the present time owes its origin 

 to Ehrlich. 



In order to attain uniformity in the comparative value of all sera, 

 Behring and Ehrlich recommended the adoption of two empirical values; 

 "the normal toxin," and the "normal curative serum." 



The normal diphtheria toxin is one which contains enough toxin in i c.c. 

 to kill 25,000 gm. of guinea-pigs or 100 guinea-pigs each weighing 250 gm. 



A normal curative serum is one of which o.i c.c. suffices to neutralize 



