CHAPTER IV. 



IMMUNITY AND SUSCEPTIBILITY. 



Immunity is resistance to disease. It is the ability of 

 an animal to protect itself against the pathogenic action 

 of bacteria. The absence or loss of this power character- 

 izes the opposed condition known as susceptibility. 



The resistance may be an active process, endogenous 

 and cytogenic in nature — active immunity ; or may be 

 hematogenous, and result from exogenous active prin- 

 ciples added to the blood — passive immunity. In the 

 active form the cells of the body may be conceived as 

 energetically engaged in destroying the bacteria, or in 

 manufacturing destructive products to act upon them. 

 In the passive form the cells take no part whatever, the 

 phenomena resulting from the presence of the experi- 

 mentally introduced active principle. 



To Infection . * 



f Active. 



Immunity, 

 Natural and i 

 Acquired. 



Cytogenic, 



Phagocytic. 



Hematogenic, 

 Alexinic. 



Passive. 



h Antitoxic. 



[ To Intoxication. 



In discussing the products of bacterial energy it has 

 been shown that the virulence of bacteria depends for 

 the most part upon certain poisonous excretory products 

 resulting from their metabolism, and that the essential 

 ' difference between pathogenic and non-pathogenic bac- 

 teria, except in a very few cases, is but a difference in their 

 products. A few bacteria, like the anthrax bacillus, pro- 



»o 



