DEFENSIVE FACTORS OF THE ANIMAL ORGANISM 247 



be incomparably more poisonous than their extracellular products 

 (toxins) . From a practical point of view, the method is of the greatest 

 importance in routine laboratory immunization against B. typhosus, 

 Vibrio cholera? asiaticse, B. pestis, and a number of other bacteria. 

 In the therapy of human disease, this method has recently come into 

 great prominence, chiefly through the work of Wright, whose inves- 

 tigations will be more fully discussed in a subsequent section. 



ACTIVE IMMUNIZATION WITH BACTERIAL PRODUCTS. Many bacteria 

 when grown in fluid media produce extracellular, soluble poisons which 

 remain in the medium after the microorganisms have been removed 

 by filtration or centrifugalization. Since the diseases caused by such 

 microorganisms are, to a large extent, due to the soluble poisons 

 excreted by them, animals can be actively immunized against this 

 class of bacteria 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 forma- 

 tion to a highly developed degree. Most important among these are 

 B. diphtherias and B. tetani. The first successful application of this 

 principle of active immunization, however, was made by Salmon and 

 Smith 15 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 

 collaborators discovered that the sera of animals immunized against 



15 Salmon and Smith, Eep. of Com. of Agri., Wash., 1885 and 1886. 



