64 INFECTION AND RESISTANCE 



processes are spoken of as "Active Immunization/' ~No protective 

 factor is conferred directly. The disease itself is inoculated, though 

 in an altered form, and the subsequent immunity is purely the result 

 of the physiological reaction occurring as the subject struggles against 

 and overcomes the injected virus, bacteria, or their products. Such 

 "Active Immunization," we shall see, is in contrast to "Passive 

 Immunization," a procedure in which the serum of an actively im- 

 munized animal is injected into another, carrying with it certain 

 substances by which protection is conferred. The recipient here is 

 passively protected by products of the active reaction which has 

 taken place in the body of the donor. 



After his success in active immunization against chicken cholera 

 Pasteur applied the principles here learned to experiments upon the 

 protection of animals against anthrax. This problem was fraught 

 with considerable difficulty because of the great virulence of the 

 anthrax bacillus. However, successful attenuation was attained by 

 a method which depended upon the cultivation of anthrax cultures at 

 temperatures above the optimum for its growth. Toussaint 14 had 

 shown that the resistance of sheep could be increased if they were 

 inoculated with blood from animals dead of anthrax after this had 

 been heated to 55 C. for 10 minutes. Toussaint's idea had been 

 that by heating the blood in this way the bacteria themselves were 

 killed. Pasteur 15 showed, however, that this was not the case, but 

 that what actually occurred was a reduction of the virulence of the 

 strain by the exposure to heat. As a matter of fact, moreover, the 

 method of Toussaint did not furnish a reliable means of attenuating 

 anthrax, and Pasteur succeeded in developing a far more satis- 

 factory procedure on which he based a practical method for the pro- 

 tective vaccination of sheep and cattle. 



His method was as follows : 16 Virulent anthrax bacilli were cul- 

 tivated at 42 to 43 C. on neutral chicken bouillon (Sobernheim 

 states that horse or beef broth or even agar answers the same pur- 

 pose). Cultivated under these conditions a gradual and progressive 

 reduction of virulence occurs. After about 12 days of such cultiva- 

 tion the culture as a rule no longer kills rabbits, but is still virulent 

 for guinea pigs and mice. After twenty-four or more days the 

 virulence for rabbits and guinea pigs is lost and mice only can be 

 killed with it. The latter the most fully attenuated strain was 

 called premier vaccin by Pasteur, and, in the immunization of cattle 

 or sheep, is first injected. After 10 or 12 days the stronger deuxieme 

 vaccin is administered. This is the method which Pasteur used in 

 his now classical experiments at Pouilly-le-Fort, in which he con- 



14 Toussaint. Compt. rend, de I'acad. des sc., 1880. 



15 Pasteur, Chamberland and Roux. Compt. rend, de Vacad. des sc., Vol. 

 91, 1881. 



16 Cited from Sobernheim. "Kraus und Levaditi Handbueh der Technik, 

 etc,," Vol. 1, 1909. 



