ANTITOXIC SERUM 467 



tinating, solvent, or other properties towards the particular 

 substance. 



After this preliminary statement in explanation we shall 

 consider the actual properties of the two classes of serum, and 

 later we shall resume the theoretical consideration. 



Antitoxic Serum. The best examples are the antitoxic 

 sera of diphtheria and tetanus, though similar principles and 

 methods are involved in the case of the anti-sera to ricin and 

 abrin, and to snake poison. We shall here speak of diphtheria 

 and tetanus. The steps in the process of preparation may be 

 said to be the following : First, the preparation of a powerful 

 toxin. Second, the estimation of the power of the toxin. Third, 

 the development of antitoxin in the blood of a suitable animal 

 by gradually increasing doses of the toxin. Fourth, the estima- 

 tion from time to time of the antitoxic power of the serum of 

 the animal thus treated. 



1. Preparation of the Toxin. The mode of preparation and 

 the conditions affecting the development of diphtheria toxin 

 have already been described (p. 362). In the case of tetanus the 

 growth takes place in glucose bouillon under an atmosphere of 

 hydrogen (vide p. 60). In either case the culture is filtered 

 through a Chamberland filter when the maximum degree of 

 toxicity has been reached. The term " toxin " is usually applied 

 for convenience to the filtered (i.e. bacterium-free) culture. 



2. Estimation of the Toxin. The power of the toxin is 

 estimated by the subcutaneous injection of varying amounts in 

 a number of guinea-pigs, and the minimum dose which will 

 produce death is thus obtained. This, of course, varies in 

 proportion to the weight of the animal, and is expressed accord- 

 ingly. In the case of diphtheria, in Ehrlich's standard, the 

 minimum lethal dose known as M.L.D. is the smallest amount 

 which will certainly cause death in a guinea-pig of 250 grms. 

 within four days. Behring uses the term " normal diphtheria 

 toxin of simple strength" (DTN 1 ), as indicating a toxin of 

 which '01 c.c. is the minimum lethal dose under these conditions. 

 A toxin of which the minimum lethal dose is '02 will be of half 

 normal strength (DTN' 5 ) ; and so on. The testing of a toxin 

 directly is a tedious process, and in actual practice, where many 

 toxins have to be dealt with, it is found more convenient to test 

 them by finding how much will be neutralised by a certain 

 amount of a standard antitoxic serum, viz. an "immunity unit" 

 (p. 468). 



3. Development of Antitoxin. The earlier experiments on 

 tetanus and diphtheria were performed on small animals, such 



