I^^iite capable of developing antitetanic power in its fluids. All that 

 is necessary is to inject into it some tetanus toxin heated to GO"* C. or 

 treated with Lugol's iodo-ioduretted solution. As the outcome of his 

 researches Vaillard concludes that the antitoxic property of the body 

 fluids "is not sufficient... for the general interpretation of acquired 

 immunity, as it cannot be demonstrated in all animals which have 

 become refractory." 



The facts I have just mentioned were demonstrated early in our 

 study of the antitoxic power of the animal organism. Since then a 

 large number of analogous data have been collected. Recently, von 

 Behring and Kitashima^ have had to abandon the immunisation of 

 monkeys against diphtheria toxin because of the poor yield in anti- [392] 

 toxin which they obtained. The blood of one of their monkeys that 

 had acquired a resisting iDower against very large doses of diphtheria 

 toxin showed only a very moderate antitoxic power. In establish- 

 ments where antitoxic serums are prepared on a large scale the 

 workers have become convinced that the yield of antitoxin has no 

 direct constant ratio to the immunity of the animal. This has been 

 demonstrated repeatedly at the stables of the Pasteur Institute. 

 Thus, of two horses, treated at the same time and in exactly the 

 same way with diphtheria toxin, one furnished a very good antitoxic 

 serum which was maintained at 200 units Ehrlich, rising up to 400 

 units, whilst the other never reached 150 units I And yet both these 

 animals possessed the same immunity against diphtheria toxin. They 

 tolerate considerable doses of toxin and react merely by a slight or 

 insignificant rise in temperature. In another series of horses, which 

 have been immunised for nearly seven years, one remained capable of 

 yielding a large quantity of antitoxin, seeing that the value of its 



1 Berlin Min. Wchnschr., 1901, S. 157. The idea of immunising monkeys against 

 diphtheria was suggested to von Behring by the fact that the immunity conferred by 

 serums was the more durable the nearer the relation between the serum used and 

 the blood of the species which receives the protective injection. Von Behring 

 supposed that the diphtheria antitoxin, introduced into the human body, would be 

 maintained there longer, if the antitoxic serum injected came from monkeys, species 

 much nearer man than is the horse, the usual source of antidiphtheria serum. The 

 immunity conferred by this horse serum is generally of very short duration. 



2 Ehrlich's antitoxic unit is adopted by most investigators not only in Germany, 

 but also in other countries. This unit corresponds to 1 c.c. of serum capable of 

 neutralising 100 lethal doses of a standard toxin, i.e. that used to establish the first 

 standard of antitoxin. The serum must be injected after being mixed in vitro with 

 the toxin. The neutralisation must be complete and give rise to no symptom of 

 intoxication. 



