248 INFECTION AND IMMUNITY 



the toxins of tetanus 16 and of diphtheria 17 bacilli would protect normal 

 animals against the harmful action of these poisons. The animals 

 thus protected obviously had taken no active part in their own defense, 

 but were protected from the action of the poison by the substances 

 transferred to them in the sera of the actively immunized animals. 

 Such immunity or protection, therefore, is a purely passive phenom- 

 enon so far as the treated animal is concerned, and the process is 

 for this reason spoken of as "passive immunization." 



Passive immunization of this description is practically applicable 

 chiefly against diseases caused by bacteria which produce powerful 

 toxins, and the sera of animals actively immunized against such toxins 

 are called antitoxic sera. In the treatment of the two diseases men- 

 tioned above, diphtheria and tetanus, the respective antitoxic sera 

 have reached broad and beneficial therapeutic application, and in- 

 numerable lives have been saved by their use. 



Passive immunization against microorganisms not characterized 

 by marked toxin formation was attempted, even before Behring's 

 discovery, by Richet and Hericourt, 18 experimenting with cocci, and 

 by Babes, 19 in the case of rabies ; and the underlying thought had 

 been the basis of Toussaint's work upon anthrax. Microorganisms, 

 however, which exert their harmful action rather by the contents 

 of the bacterial cells than by secreted, soluble toxins, do not, so far 

 as is known, produce antitoxins in the sera of immunized animals. 

 The substances which they call forth in the process are directed against 

 the invading organisms themselves in that they possess the power 

 of destroying or of causing dissolution of the specific germs used in 

 their production. 



Such antibacterial sera are extensively used in the laboratory in 

 the passive immunization of animals against a large number of germs, 

 and are fairly effectual when used before, at the same time with, 

 or soon after, infection. Their therapeutic use in human disease, 

 however, has, up to the present time, been disappointing and their 

 prophylactic and curative action has been almost invariably ineffectual 

 or feeble at best, except when the antibacterial sera could be brought 

 in direct contact with the germs, in closed cavities or localized lesions. 



16 v. Bearing and Kitasato, Dent. med. Woeh., 49, 1890. 



17 v. Bearing and Wernicke, Zeit. f . Hyg., 1892. 



18 Richet et Hericourt, Compt. rend, de Paead. dos sei., 1888. 



19 Babes et Lepp, Ann. de 1'inst. Pasteur, 1889. 



