232 GENERAt BIOLOGY OF MICRO-ORGANISMS 



passive immunity has been demonstrated in anumber of diseases. In 

 some instances the effect of the serum is antitoxic (diphtheria and te- 

 tanus) , in others it is bafcteriolytic (cholera;) , while in other instances 

 the nature of the dominant antibodies is not definitely known. 



Combined Active and Passive Immuiiity. — Various procedures 

 have been devised to produce immunity by introducing at, or 

 nearly at, the same time the infectious agent or its products and 

 the serum of an immune animal containing protective substances. 

 The combination of immune blood with virus of full strength is 

 used in immunizing animals against rinderpest, foot-and-mouth 

 disease and hog cholera, all being diseases due to filterable agents; 

 arid also in immunizing hogs against hog erysipelas (B. rhusio- 

 pathice). The combined injection of attenuated virus and immune 

 serum is employed especially in Sobernheim's method of preventive 

 inoculation against anthrax. Besredka has employed dead 

 bacteria combined with their specific immune serum in immunizing 

 against typhoid fever, plague and cholera. 



The Mechanisms of Immunity. — Certain biological factors 

 in the phenomenon of immunity are now clearly recognizable 

 and readily demonstrable. The activity of the phagocytes, first 

 emphasized by Metchnikoff and beHeved by him to be the sole 

 important factor in the defense of the body, is easily observed 

 in immunity to many diseases. The dependence of phagocytic 

 activity upon dissolved substances in the body fluids (opsonins) 

 is also demonstrated beyond doubt. Phagocytosis is, perhaps, 

 the factor of most general operation in immunity to all sorts of 

 disease. The antitoxins stand forth prominently as powerful 

 factors in immunity to two important diseases, diphtheria and 

 tetanus, and the bacteriolysins are undoubtedly of greatest im- 

 portance in the case of Asiatic cholera, and probably also in ty- 

 phoid and plague. In most instances the immunity seems to 

 depend upon several different factors, phagocytosis, opsonins, 

 bacteriolysins, antitoxins, and perhaps substances of unknown 

 nature. In some instances of immunity there is no particular 

 excess of these immune bodies demonstrable in the blood, and 



