DIPHTHERIA ANTITOXIN 77 



If an animal, e.g., a goat is injected with a sublethal dose of 

 Active Im- diphtheria toxin and after the lapse of a certain period of time 

 munization it is reinjected with a lethal dose, the animal remains alive, 

 against In fact it may receive numerous fatal doses, and still survive. 

 a Toxin. This experiment is the simplest in active immunization 

 against a toxin. An examination of the blood serum of the 

 immunized animal will disclose very readily what has taken place. If 

 this serum is mixed with a fatal dose of toxin and the mixture inoculated 

 into a normal guinea-pig, the latter remains alive and perfectly active. 

 The serum of the immunized animal therefore contains a 

 Antitoxin, protective agent which is directed against the toxin and de- 

 stroys its activity; hence the name antitoxin. But the anti- 

 toxin is specific, i.e., diphtheria antitoxin neutralizes only diphtheria 

 toxin and not tetanus. The recognition of these facts and those 

 heretofore mentioned, and the recommendation of the therapeutic use 

 of diphtheria serum belong entirely to v. Behring; righteously may he be 

 called the father of serum therapy. 



Although theoretically the serum of any animal immunized with diphtheria toxin 

 can serve as a curative serum for diphtheria, practical experience has taught that it is 

 best to employ horses for this purpose. For laboratory experiments goats should be 

 the animals of choice. It is advisable to use the above animals for the reason that 

 largerquantities of serum are obtained and furthermore because it has been found impos- 

 sible to immunize guinea-pigs with previously unchanged diphtheria toxin even if the initial 

 dosage is the smallest subdivision of the minimal lethal dose. Behring and Kitashima 

 showed that after repeated injections of very minute doses they were able to kill guinea- 

 pigs even with 1/400 of the dosis letalis minima. This is but another example of an 

 effect just opposite to that of immunity and known as hyper susceptibility or hypersensi- 

 tiveness, which has already been described in the chapter on tuberculin therapy. If, 

 however, it is desired to immunize guinea-pigs, a modified form of the diphtheria toxin 

 must be employed for the first injections. Several modifications are feasible. Behring 

 and Kitasato added iodin trichlorid to the toxin while Roux and Martin chose Lugol's 

 solution; C. Frankel heated it to 60 C, and Behring advocated the so-called "simul- 

 taneous method" (of special aid in tetanus toxin), where mixtures of toxin and anti- 

 toxin are injected and gradually the quotient of the latter is diminished until finally it 

 is entirely omitted. If the animals have borne the first inoculations of the modified 

 toxin without any ill effects, one may then use the unmodified toxin. 



In contrast to small animals, horses can be immunized with unmodified diphtheria 

 toxin right from the start. Nevertheless great care must also here be exercised. Cer- 

 tain it is, that less risk is run in the employment, with even the larger animals, of a 

 modified toxin. For the production of a good diphtheria serum, healthy horses about 

 five to six years old are used and gradually increasing amounts of diphtheria toxin are 

 injected subcutaneously or even intravenously; thus agreeing with Ehrlich's findings 

 to the effect that the antitoxin content of a serum can be raised by successively increas- 

 ing the amount of toxin injected. As far as the efficiency of the immune serum is 

 concerned, it is entirely dependent on the animal. Horses vary greatly in their 

 individual predisposition toward the production of an effective serum; some animals 



