496 Chapter XV 



repeated doses of the toxin manufactured by the diphtheria bacillus. 

 [519] The serums thus obtained were first tested as to their protective, 

 antitoxic, and curative action on guinea-pigs, animals very susceptible 

 to diphtheria. The necessity of finding some means of measuring 

 the strength of the serum soon arose. Von Behring and Wernicke 

 at first standardised it on the basis of the number of grammes of 

 guinea-pig which could be protected by one gramme of serum. Later, 

 von Behring 1 introduced the principle of the " normal serum," that 

 is to say, a serum of which O'l c.c., mixed with 10 lethal doses of 

 diphtheria toxin, is capable of preventing every morbid symptom in 

 a guinea-pig weighing 300 to 400 grammes. 



Ehrlich 2 perfected this method in the following way : to tubes, 

 each containing 10 lethal doses of a standard toxin, are added 

 different amounts of serum. These mixtures are brought to the 

 same volume of 4 c.c. by the addition of physiological saline solution, 

 and each is immediately injected below the skin of a guinea-pig. If 

 O'l c.c. of a serum completely neutralises the 10 lethal doses of toxin, 

 the serum retains its name of normal serum ; in the case where 0*05 c.c. 

 is sufficient to bring about the same result the serum is designated 

 double normal serum. When O'OOl c.c. gives the same results, a 

 hundred times normal serum, and so on. A cubic centimetre of 

 normal serum (that is to say a dose capable of neutralising 100 lethal 

 doses of standard toxin) constitutes an " immunising unit " (Immuni- 

 sirungseinheit (I.E.) of Ehrlich). As it was soon recognised that toxins, 

 even when kept under the best conditions, lose more or less of their 

 toxic power, Ehrlich had to modify his method of standardising 

 serum. He now makes use of a standard antidiphtheria serum, kept 

 in a dry condition, which is much more constant than are the toxins. 

 Solutions of this standard serum are prepared and compared with 

 the serum whose strength has to be determined. Ehrlich has given a 

 detailed description of the method of procedure required to obtain 

 exact results. 



At the Pasteur Institute Ehrlich's method has been adopted, 

 supplemented however by another test for the estimation of the 

 strength of antidiphtheria serums, a method allied to von Behriug's 

 old method. Various doses of the serum to be examined are injected 

 subcutaneously into guinea-pigs, and 24 hours later these guinea-pigs 



1 Deutsche med. Wchnschr., Leipzig, 1893, S. 390. 



1 Ehrlich, Kossel u. Wassermann, Deutsche med. Wchnschr., Leipzig, 1894, S. 353; 

 Klin. Jahrb., Berlin, 1897, Bd. vi. 



