174 



THE VARIOUS TYPES OF IMMUNITY 



able to set free the protein poison responsible for the symptoms of the 

 infection. 



Natural immunity to any infection, according to Vaughan's theory, is 

 explained as being due to an inability of the infecting agent to grow in 

 the animal body. 



Acquired immunity, due to recovery from an infection or occurring as 

 a result of vaccination, is regarded as the outcome of the development 

 in the body, during the course of the infective process, of a specific fer- 

 ment that, on renewed exposure, immediately destroys the infection. 

 The vaccine is the same protein that causes the disease, so modified that 

 it will not produce the disease, but yet so little altered that it will stim- 

 ulate the body-cells to form a specific ferment that will promptly and 

 quickly destroy the infecting agent on exposure. 



The various kinds of immunity and the factors probably concerned 

 in their production, may be summarized as follows : 



1. Due to non-specific factors: 



1. Barrier of epithelium. 



2. Various secretions. 



3. A particular route of infection may be nec- 



essary, aided by the biologic nature of 

 the invading bacterium. 



Natural Immunity 



Acquired Immunity 



2. Due to local tissue immunity and selective ac- 



tion of micro-parasites for certain tissues. 



3. Due to phagocytosis. 



4. Due to lack of suitable receptors of body-cells 



for a particular bacterium. 



5. Due to natural antitoxins. 



6. Due to natural bacteriolysins. 



7. Due to anti-aggressins. 



8. Due to lack of suitable food material athrep- 



sia. 



Active (accidentally or } 1. Antitoxic. 



artificially acquired) j 2. Antibacterial. 



1. Antitoxic. 



2. Antibacterial. 



Passive 



