CHAPTER VI. 

 IMMUNITY. 



DEFINITION AND SCOPE.— Immunity is that phe- 

 nomenon by which an animal or whole class of animals ex- 

 hibits such resistance to an infectious disease as to withstand 

 it, while under the same conditions another animal or class of 

 animals will yield. In its broad sense immunity is exemption 

 from disease, but it is usually limited to diseases of parasitic 

 origin, either animal or vegetable. There is no such thing as 

 immunity from accident or death. When a pathogenic and 

 parasitic microbe invades the body of an animal, complicated 

 phenomena ensue, the whole process being conventionally 

 looked upon in the nature of a conflict. On the one hand, 

 offensive weapons brought into action by the microbe enable 

 it to pass a phase of its existence in the body before it makes 

 exit. On the other hand, the host, by various defensive me- 

 chanisms, seeks to protect its cellular units from the advanc- 

 ing microbe. From the character of the vital processes of 

 both the invader and the invaded, one can easily comprehend 

 that the phenomena of infection and immunity are at once 

 complex. 



Immunity is not a theory. It is a science, although not 

 perfectly understood ; but relatively speaking, we have a bet- 

 ter knowledge of the action of the protective substances in 

 the blood, — antitoxins, etc., — than of the physiological action 

 of quinine or aconite. The only real theory concerning im- 

 munity, and this will be taken up in detail later, is in regard 

 to the manner in which these substances are formed and in 

 regard to their mode of action. That the antitoxins and 

 allied bodies are of great therapeutic, diagnostic and prophy- 



IBO 



