CHAPTER II. 



IMMUNITY 



Immunity may be defined as, non-susceptibility to a disease, 

 or as the ability to resist the action of the causes of the disease. 

 The body may be immune because of inherited properties or 

 because it has become so during life. Immunity because of in- 

 herited properties is called "natural" immunity, while the im- 

 munity acquired during life is called "acquired" immunity. 



Natural immunity is demonstrated by the non-susceptibility 

 of the hen to the action of the tetanus bacillus. It is an im- 

 munity of race or species. This immunity at times may be re- 

 duced or removed by hunger, exhaustion, exposure to cold, etc. 

 Certain closely related races or species of animals sometimes show 

 a natural immunity and at times a natural susceptibility to the 

 same infecting agent. This non-susceptibility is frequently called 

 a natural resistance and at times is only an apparent immunity, 

 depending in these cases on the common natural barrier to the 

 entrance and development of disease producing organisms. Again, 

 what may be regarded as a natural immunity is, in part at least, 

 only a resistence to infection due to the inability of organisms to 

 reach viable tissues. This is the case when the acidity of the 

 stomach is sufficient to kill cholera organisms before they reach 

 the epithelium of the intestine. 



Acquired immunity only results after a pathological condi- 

 tion exists or has existed. The individual becomes immune, 

 because he has survived a natural course of the disease, as is the 

 case following an attack of scarlet fever; because he has gone 

 through a modified form of the disease, as is the case in vaccination 

 against small-pox; or, because he receives substances prepared by 

 some other individual or animal that has gone through a natural 



