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 

 or inborn 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 bvJahoratory methods of inocu- 

 lation, this is termed properly an acquired immunity, and injthe former 

 case is anna.t.ii rally acquired immunity and in the second an artificially 

 acquired immunity. 



Immunity then 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, which may be regarded as 

 accidentally acquired, or in response to the stimulus of the injection of 

 the organism or its products, we have developed in the man so infected 

 or 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, precipitating 

 power, opsom'c power, or bacteriolytic power. The term antibody is 

 also applied. All four 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 

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