Passive Acquired Immunity 97 



quarter evil, similarly to immunize animals against these 

 respective infections. 



The subject was much further elaborated by Roux and 

 Yersin* in their experiments with diphtheria toxin, by 

 Behringf in his early studies of diphtheria, and by Kita- 

 satoj in his experiments with tetanus. 



These early experiments opened a wide field now thor- 

 oughly investigated, so that we know that the dead as 

 well as the living micro-organisms, and the products as 

 well as the living or dead bacteria of most of the infectious 

 diseases, when properly introduced into animals, can in- 

 crease the resisting power. 



(B) Passive Acquired Immunity. Passive immunity is 

 always acquired, never natural. It depends upon defensive 

 factors not originating in the animal protected, but arti- 

 ficially or experimentally supplied to it. The fundamental 

 principle is simple and has become the basis of serum thera- 

 peutics. If the immunized animal generates factors by 

 which the infecting bacteria can be destroyed or the activity 

 of their products overcome in its body, cannot these fac- 

 tors be removed and the benefit they confer transferred to 

 another animal? 



The first experiments in this direction seem to have 

 been made by Babes and Lepp, who found that the 

 blood-serum of animals immunized to rabies showed a 

 defensive power when injected into other animals. Ogata 

 and Jasuhara|| found that the subcutaneous injection of 

 blood-serum from an animal immunized against anthrax 

 enabled the injected animals successfully to resist infection. 

 Behring and Kitasato** found that the blood-serums of 

 animals immunized against diphtheria and tetanus, when 

 mixed with Cultures of these respective bacilli, neutralized 

 their power to produce disease. Kitasatoff found that if 

 mice were inoculated with tetanus bacilli, they could be 

 saved from the fatal infection by the intra-abdominal in- 

 jection of some blood-serum from a mouse immunized 



* " Ann. de 1'Inst. Pasteur," n, 1888, p. 629. 



f " Deutsche med. Wochenschrift, 1890, No. 50. 



J " Zeitschrift fur Hygiene," x, 1891, p. 267. 



" Annales de 1'Inst. Pasteur," 1889, v l- m - 



|| "Centralbl. f. Bakt.," etc., ix, p. 25, 1890. 

 ** " Deutsche med. Woch.," 1890, No. 49. 

 ft " Zeitschrift fur Hygiene," 1892, xn, p. 256. 



