ACQUIRED IMMUNITY 65 



tion. 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 the- 

 oretical 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 bacil- 

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

 u 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 

 minutes. 



(2) Attenuation by prolonged cultivation of the bacteria at 

 temperatures above the optimum for their growth. This is illus- 

 trated by Pasteur's anthrax immunization as described in the pre- 

 ceding paragraphs. 



(3) Attenuation by passage through animals. Examples of 

 this are Pasteur's experiments with the "rouget" organism, in which 

 passage through rabbits diminished the virulence for hogs. The 

 attenuation of rabic virus by passage through monkeys is another 

 instance, and Jennerian vaccination is also an example of this, al- 

 though here the attenuation by passage through cattle is attained 

 naturally and not by experimental procedures. Based on the same 

 principle is Behr ing'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 f oducts. This is the 

 method first employed by Pasteur in chicken cnolera, as described 

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

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

 virulence of many of these bacteria unless special methods of culti- 

 vation or passage through animals are practiced. Pasteur believed 

 that free access of oxygen to the cultures increases the rapidity of 

 the attenuation. 



(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. 



