Artificial immunity against toxins 375 



chemio taxis for micro-organisms during the acquisition of anti-in- 

 fective immunity. 



Later, von Behring^ changed his opinion. Whilst still accepting 

 the change of cellular susceptibility in the direction of hypersensi- 

 tiveness in animals immunised against toxins, he refused to admit 

 the change in the opposite direction. The cells, according to him, 

 never lose any of their susceptibility, so that acquired immunity 

 against toxins cannot be obtained otherwise than by means of[394j 

 antitoxins capable of neutralising the poison in a susceptible or 

 hypersusceptible animal. This new theory von Behring upheld 

 in several papers and it is met with in his most recent publications. 

 Nevertheless, certain well-established facts compel us to accept an 

 immunity against toxins as coming about as the result of a diminu- 

 tion of the susceptibility of the vaccinated animal. Parallel with his 

 researches on the increase of the susceptibility of guinea-pigs to 

 tetanus toxin, researches discussed above, Knorr^ describes analogous 

 experiments on rabbits. When these animals are injected with 

 fractions of the minimal lethal dose, frequently repeated, the 

 rabbit not only does not become hypersusceptible to tetanus but 

 exhibits a greater and greater insusceptibility. Whilst guinea- 

 pigs, treated according to this method, die from tetanus before 

 they have reached the minimal lethal dose, rabbits, as the result of 

 frequent injections of small quantities of tetanus toxin, become 

 capable of resisting five times the lethal dose (for normal rabbits) 

 without exhibiting the slightest symptom of illness. Against the 

 attribution of this result to the acquired insusceptibility of the living 

 animals it might be objected that the immunity, in this case, may 

 depend on the antitoxic power of the fluids of the body, developed 

 with great rapidity. Such an objection cannot be raised in the case 

 of horses which become insusceptible to toxins after a long period 

 of vaccination. The horse whose history was given above, when 

 discussing the diminution of antitoxic power, may serve as an ex- 

 ample. At the commencement of its vaccinal period, in 1894, it 

 reacted to the injection of 10 c.c. of diphtheria toxin by a rise of 

 temperature of 1"* C. Four years later, when its blood had become 

 very antitoxic (350 units per c.c), it was necessary to inject 350 c.c. 



^ Article "Immunitat" in Eulenburg's Realencyclopddie, III^ Aufl., Wien, 1896 ; 

 see also his "AUgemeine Therapie d. Infectionskrankheiten," in Eulenburg u- 

 Samuel's " Lehrb. d. allg. Therapie," Berlin u. Wien, 1899, Bd. m, SS. 996, 997. 



2 Op. cit. supra p. 370, S. 19. 



