CHAPTER XII. 

 PRACTICAL METHODS IN IMMUNITY. 



THAT which prevents the gaining of a foothold by disease organisms 

 in the animal body or which neutralizes their harmful products or 

 destroys the parasites is termed immunity. In the main, the question 

 of immunity hinges on the powers of resistance of the human body 

 and the aggressiveness or virulence of the invading organism. It 

 must always be kept in mind that immunity is only relative; thus the 

 fowl, which is practically immune to tetanus, may be made to suc- 

 cumb by reducing its resistance by refrigeration or by increasing the 

 amount of poison introduced. The insusceptibility which the fowl 

 has to tetanus or which man has to many diseases of animals is best 

 termed inherent immunity, and is at present only a subject of theo- 

 retical interest. When immunity to a given disease is obtained as a 

 result of an attack of the disease in question or by laboratory methods 

 of inoculation, this is termed properly an acquired immunity, and in 

 the former case is a naturally acquired immunity or "natural im- 

 munity" and in the second is an artificially acquired immunity or 

 " artificial immunity." 



As a result of an attack of a disease or in response to the stimulus 

 of the injection of the organism or its products, we have developed in 

 the man so injected certain specific antagonistic properties to that 

 organism, which are usually demonstrable in the blood serum or 

 other body fluids, and to which we apply the terms agglutinating 

 power, opsonic power or bacteriolytic power. The term antibody is 

 also applied. All three powers may be present together in equal or in 

 varying degree or one or more may be absent. By agglutinating 

 power we mean that which causes evenly distributed organisms to 

 come together and form clumps. By opsonic power we mean that 

 which so alters the resistance of bacteria that the phagocytes ingest 

 them. By bacteriolytic power we mean that which brings about 



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