196 INFECTION AND IMMUNITY 



teria by the inoculation of gradually increasing doses of the specific 

 poison or toxin. This method is naturally most successful against those 

 microorganisms which possess the power of toxin formation to a highly 

 developed degree. Most important among these are B. diphtherise 

 and B. tetani. The first successful application of this principle of active 

 immunization, however, was made by Salmon and Smith' in the case of 

 hog cholera. 



PASSIVE IMMUNITY 



In Pasteur's basic experiments, as in those of the other scientists who 

 followed in his footsteps, the methods of immunization were based upon 

 the development of a high resistance in the treated subject by virtue of 

 its own physiological activities. This process we have spoken of as 

 " active immunization " and it is self-evident that a method of this kind 

 can, in the treatment of disease, be employed prophylactically only 

 against possible infection, or in localized acute infections, or at the 

 beginning of a long period of incubation before actual symptoms have 

 appeared, as in rabies or in chronic conditions in which the infection is 

 not of a severe or acute nature. 



A new and therapeutically more hopeful direction was given to the 

 study of immunity when, in 1890 and 1892, v. Behring and his collabora- 

 tors discovered that the sera of animals immunized against the toxins 

 of tetanus ^ and of diphtheria ^ bacilli would protect normal animals 

 against the harmful action of these poisons. The animals thus pro- 

 tected obviously had taken no active part in their own defense, but 

 were protected from the action of the poison by the substances trans- 

 ferred to them in the sera of the actively immunized animals. Such 

 immunity or protection, therefore, is a purely passive phenomenon 

 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 



1 Salmon and Smith, Rep. of Com. of Agri., Wash., 1885 and 1886. 



2 V. Behring and Kitasato, Deut. med. Woch., 49, 1890. 

 'V, Behrinq and Wernicke, Zeit. f. Hyg., 1892. 



