IMMUNITY AND HYPE INSUSCEPTIBILITY 225 



In some instances the effect of the serum is antitoxic (diphtheria 

 and tetanus), in others it is bacteriolytic (cholera), while in 

 other instances the nature of the dominant antibodies is not 

 definitely known. 



Combined Active and Passive Immunity. 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; and also in immunizing hogs against hog erysipelas 

 (B. rhusiopathice). 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 believed 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 

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