DEFENSIVE FACTORS OF THE ANIMAL ORGANISM 245 



Whatever the method employed, the immunized individuals gain 

 their power of resistance by the unaided reactions of their own tissues. 

 They themselves take an active physiological part in the acquisition 

 of this new property of immunity. For this reason, Ehrlich has aptly 

 termed these processes * ' active immunization. ' ' 



There are various methods by which this can be accomplished, all 

 of which were, in actual application or in principle, discovered by 

 Pasteur and his associates, and can be best reviewed by a study of 

 their work. 



ACTIVE IMMUNIZATION WITH ATTENUATED CULTURES. In the 

 course of his experiments upon chicken cholera, as mentioned above, 

 Pasteur 6 accidentally discovered that the virulence of the bacilli of 

 this disease was greatly reduced by prolonged cultivation upon artifi- 

 cial -media. This was especially noticeable in broth cultures which 

 had been stored for long periods without transplantation. By repeated 

 injections of such cultures into fowl, he succeeded in rendering the 

 animals immune against subsequent inoculations with lethal doses of 

 fully virulent strains. 



During the same year, 1880, in which Pasteur published his ob- 

 servations on chicken cholera, Toussaint 7 succeeded in immunizing 

 sheep against anthrax by inoculating them with blood from infected 

 animals, defibrinated and heated to 55 C. for ten minutes. Toussaint 

 wrongly believed, however, that the blood which had been used in 

 his immunizations was free from living bacteria. In repeating this 

 work Pasteur showed that the protection in Toussaint 's cases was 

 conferred by living bacteria, the virulence of which had been reduced 

 by their subjection to heat. 



In following out the suggestions offered by these experiments, 

 Pasteur 8 discovered that he could reduce the virulence of anthrax 

 bacilli much more reliably than by Toussaint 's method, by cultivating 

 the organisms at increased temperatures (42 to 43 C.). By this 

 process of attenuation he was able to produce "vaccines" of roughly 

 measurable strength, with which he succeeded in immunizing sheep 

 and cattle. A successful demonstration of his discovery was made 

 by him at Pouilly-le-Fort, soon after, upon a large number of animals 

 and before a commission of professional men. 



6 Pasteur, Compt. rend, do 1'acad. des sci., 1880, t. xc. 



7 Toussaint, Compt. rend, de 1'acad. des sci., 1880, t. xei. 



* Pasteur, Chamberland et Eoux, Compt. rend, de 1'acad. des sci., 1881, t. xcii. 



