98 INFECTION AND IMMUNITY. 



Immunity is, in general terms, certainly produced by certain 

 secretions formed in the animal's body, and secreted by it to 

 protect itself from the attack of the invading bacteria, and dis- 

 tributed in all the tissues, but found especially in the serum of 

 the blood. 



IV. The chain-theory (Ehrlich's) claims that this immuniz- 

 ing substance is developed on account of the fact that the poi- 

 sonous substances introduced by the microbes or the secretions 

 of the microbes in the animal body combine with certain 

 elements of the tissue and destroy them, subtracting them 

 from other elements with which they were naturally in com- 

 bination ; this stimulates the natural resistance of the tissues 

 and causes an increased production of the substances attacked 

 by the bacteria in such a way that an overproduction results, 

 and this makes the animal more resistant to the further intro- 

 duction of the poison. This is certainly the most plausible 

 explanation of immunity offered to this day. 



A passing remark, however, may only be offered on this 

 subject, and those who are interested must be directed to con- 

 sult larger works, in which these views are explained at 

 length. 



QUESTIONS. 



What is infection ? 



How are bacteria called which produce disease in animals ? 



How is the action of pathogenic bacteria on the animal body explained ? 



When is a disease said to be septicsemic ? When is it toxseniic ? 



Name the three modes by which the animal body may be infected. 



What conditions favor infection ? 



What conditions in the infecting material increase its power? 



What conditions in the animal increase the rapidity of infection ? 



What part does the quantity of bacteria introduced in the inoculation play 

 in the infection? 



What is meant by attenuation ? 



What conditions of the cultures make the bacteria more virulent? 



What is the effect of passing for a number of generations pathogenic bac- 

 teria through artificial media? 



What part does the mode of introduction of the bacteria in the animal body 

 play in infection? 



What is meant by double infection ? 



What is immunity ? 



What is meant by natural immunity ? 



What is meant by acquired immunity ? 



Give some examples of natural immunity? 



What produces acquired immunity in animals? 



