ACQUIRED IMMUNITY 65 



vinced a hostile audience of the efficacy of his immunization. Sheep 

 were protected in the manner indicated, and 14 days after the last 

 injection a fully virulent culture was inoculated and the animals 

 found capable of successfully resisting it. 



In the train of this work many other methods of producing active 

 immunity have been devised all of them of considerable theoretical 

 interest and many of them practically adapted to some special case. 

 We may conveniently classify these methods as follows : 



I. IMMUNIZATION WITH LIVING BUT ATTENUATED CULTURES 



(1) Methods in which the attenuation is obtained by heating. 

 This is the method of Toussaint as outlined above, in which anthrax 

 blood was heated to 55 C. for 10 minutes, and is probably the least 

 efficient or reliable method for the attenuation of the anthrax bacillus. 

 It has been applied to rabies by Babes (cited from Kraus in "Kraus 

 u. Levaditi Handbuch, etc.," Vol. 1, p. 708), who attenuated the 

 virus by heating to 58 C. for periods varying from 2 to 40 min- 

 utes. 



(2) Attenuation by prolonged cultivation of the bacteria at tem- 

 peratures above the optimum for their growth. This is illustrated by 

 Pasteur's anthrax immunization as described in the preceding para- 

 graphs. 



(3) Attenuation by passage through animals. Examples of this 

 are Pasteur's experiments with the a rouget" organism, in which pas- 

 sage through rabbits diminished the virulence for hogs. The attenu- 

 ation of rabic virus by passage through monkeys is another instance, 

 and Jennerian vaccination is also an example of this, although here 

 the attenuation by passage through cattle is attained naturally and 

 not by experimental procedures. Based *on the same principle is 

 Behring's 17 method 18 of immunizing cattle against tuberculosis by 

 inoculating them with tubercle bacilli of the human type. 



(4) Attenuation by prolonged growth of bacteria on artificial 

 media in the presence of their own metabolic products. This is the 

 method first employed by Pasteur in chicken cholera, as described 

 above, and is applicable to many organisms, such as pneumococci, 

 streptococci, and others. In fact, it is difficult to maintain the viru- 

 lence of many of these bacteria unless special methods of cultivation 

 or passage through animals are practiced. Pasteur believed that free 

 access of oxygen to the cultures increases the rapidity of the attenua- 

 tion. 



(5) Attenuation by drying. The classical example for this 

 method is the Pasteur method of prophylactic immunization against 



17 Behring. "Therapie der Gegenwart," April, 1907. 



18 See also Romer, "Kraus u. Levaditi Handbuch," 1st Suppl., p. 310. 



