DEFENSIVE FACTORS OF THE ANIMAL ORGANISM . 193 



possibility of artifically bestowing immunity without inflicting the 

 dangers of the fully potent infective agent. The first observation which, 

 made by him purely accidentally, inspired the hope of the achievement 

 of such a result, occurred during his experiments with chicken cholera. 

 The failure of animals to die after inoculation with an old culture of the 

 bacilli of chicken cholera, fully potent but a few weeks previously, 

 pointed to the attenuation of these bacilli by their prolonged cultivation 

 without transplantation. With this observation as a point of departure 

 he carried out a series of investigations with the purpose of discovering 

 a method of so weakening or attenuating various incitants of disease 

 that they could be introduced into susceptible individuals without en- 

 dangering life and yet without losing their property of conferring pro- 

 tection. The brilliant results achieved by Jenner, many years before, 

 in protecting agains^ smallpox by inoculating with the entirely innocu- 

 ous products of the pustules of cowpox furnished an analogy which 

 gave much encouraging support to this prospect. 



The experimental work which Pasteur carried out to solve this prob- 

 lem not only reaped a rich harvest of facts, but gave to science the first 

 and brilliant examples of the application of exact laboratory methods to 

 problems of immunity. 



ACTIVE IMMUNITY 



Active Artificial Immunity. The process of conferring protection 

 by treatment with either an attenuated form or a sublethal quantity 

 of the infectious agent of a disease, or its products, is spoken of as " active 

 immunization." 



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 * 



1 Pasteur, Compt. rend, de Pacad. des sci., 1880, t. xc. 



