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 de- 

 stroys 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 succumb 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 theoretical 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 immunity" and in the second 

 is an artificially acquired immunity or " artificial immunity." 



Immunity may be divided into that which is inherent and that which is acquired. 

 Inherent immunity is such as is observed in the resistance of Algerian sheep to 

 anthrax (ordinary sheep are very susceptible) or the fowl to tetanus and is of interest 

 theoretically rather than practically. Acquired immunity may be brought about 

 naturally as by an attack of a disease or artificially by laboratory measures. 



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

 of the injection of the organisms 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 vary- 

 ing 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 



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