4/0 IMMUNITY. 



corresponding bacterium, and may even produce alteration in 

 its structure. It is manifest that such a serum differs funda- 

 mentally in its point of attack, so to speak, from an antitoxic 

 serum. And it is to be noted that the nature of the serum 

 corresponds in a wonderful way with the requirements of the 

 organism, that is, is antitoxic in such diseases as diphtheria and 

 tetanus where toxic action is at its maximum, and bactericidal 

 where the rapid multiplication of the bacteria in the tissues is 

 the outstanding feature. It must not be supposed, however, 

 that a serum must be purely antitoxic or purely antibacterial 

 according to the method by which it is prepared. For example, 

 an antitoxic serum can be obtained by injecting living diphtheria 

 bacilli into the tissues of an animal, the antitoxic property being 

 in all probability developed by means of toxins formed by the 

 bacilli within the body. Having given this explanation, we shall 

 consider the two kinds of serum separately. 



Antitoxic Serum. The best examples are the antitoxic 

 sera of diphtheria and tetanus, though similar principles and 

 methods are involved in the preparation of the sera protective 

 against ricin and abrin, and against 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 prepa- 

 ration 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 estimation 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. 366). In the case of tetanus 

 the growth takes place in glucose bouillon under an atmosphere 

 of hydrogen (vide p. 63). 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 es- 

 timated 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 accordingly. In the 



