THEORIES OF IMMUNITY 487 



serum of an animal which has recovered from the disease or which 

 has been highly immunized to the virus or cause. 



Among the theories elaborated to explain acquired immunity the 

 following should be nientioned : 



1. The Exhaustion Theory. This was Pasteur's own theory of 

 the mechanism of acquired immunity. He believed that the living- 

 microorganism of a disease in an infected animal used up some sub- 

 stance essential to its existence, so that for lack of proper nourish- 

 ment, the organisms or bacteria were forced to retire, the soil being 

 unfit for further development. 



2. The Retention Theory. Chauveau, on the other hand, con- 

 sidered it more probable that the microorganism after having lived in 

 the body of an infected animal, produced substances which, accumu- 

 lating in the blood, had an inhibitory action on the bacteria and 

 caused them to die off. This was really just the opposite to the 

 belief of Pasteur. 



3. The Phagocytosis Theory. This was the product of the 

 genius of MetchnikofF. He showed that certain cells of the body, par- 

 ticularly the polynuclear leucocytes, were very active in the defense 



' of the body against invasion by microorganisms. Abcording to this 

 theory, these cells ingested or devoured invading bacteria in a manner 

 similar to that of an ameba ingesting a food particle. After being 

 ingested, the cells disposed of the bacteria by intracellular digestion 

 through the agency of ferments known as " cytases." To cells which 

 possess such protective properties the term " phagocyte " is applied 

 and to the process of actual ingestion of the organisms, the term 

 " phagocytosis." This theory was still further elaborated by Wright 

 and Douglas, who showed that certain of the body fluids, notably the 

 blood serum, possessed the property of lowering the resistance of 

 bacteria, rendering them more susceptible to phagocytosis. To that 

 substance which rendered bacteria more " palatable " to the phagocyte 

 Wright gave the name " opsonin." 



4. The Humeral Theory. This theory is based on the observa- 

 tions of Buchner and others that blood serum has the power of de- 

 stroying a certain number of bacteria when introduced into it. 



5. Ehrlich's Side-chain Theory. This is really one explana- 

 tion of the Humeral Theory of immunity but was advanced in 1885 

 to explain the process of nutrition. Ehrlich believed that cells have 

 two important functions: (a) a special physiologic function, as that 

 of a gland cell to secrete ; (b) a function of nutrition and presiding 

 over the process of waste and repair. The second function or that 

 concerned with nutrition is of more importance in relation to im- 

 munity. According to his belief the cells have a number of chemical 

 groups or side chains. Immunity depends upon the presence or ab- 

 sence of certain substances which he calls receptors or lateral chains^ 



Kinds of Immunity. • Toxic Immunity is where the disease is. 



