THE. MECHANISM OF ACTIVE IMMUNITY 481 



the body cells to set up a resistance and produce an active 

 immunity. 



Vaccination against typhoid,, The principle underlying vacci- 

 nation against typhoid is that of working up an active immunity 

 by introducing into the body large numbers of dead typhoid germs. 

 The presence of the dead bacilli stimulates the blood to make 

 antibodies and thus an active immunity is acquired. This im- 

 munity protects the person against the invasion of living germs. 



During the Spanish-American War in the army of 107,000 men 

 more than 20,000 were disabled with typhoid. Since 1914, after 

 vaccination against typhoid was introduced, the disease has been 

 almost stamped out in the army, and the death rate for the entire 

 country has been so much reduced that it is now a disease of 

 relatively little importance. 



The Widal test, by means of which it is possible to determine at 

 once whether a person has typhoid, has been described on page 390. 



The mechanism of active immunity. Active immunity is 

 thus brought about in a number of different ways : by the intro- 

 duction of living organisms, by the introduction of attenuated or 

 weakened organisms, by the introduction of dead organisms, and 

 by the introduction of extracts containing the products of bacteria. 

 All of these substances may be called vaccines. 



The underlying principle is the same in all cases ; certain cells of 

 the body are roused or activated to form antibodies, and the invad- 

 ing organisms are destroyed and their toxins neutralized. These 

 conditions are brought about through the work of the lysins, 

 precipitins, agglutinins, opsonins, and phagocytes already men- 

 tioned in Unit XIII. You should read that unit carefully again 

 in connection with the present unit. 



Other vaccines are made and used successfully against boils, 

 another against paratyphoid, and still others for plague and for 

 cholera. When tests show sensitiveness to certain pollens, serums 

 are made from them and a certain amount of immunity from 

 hay fever is thus received. But we are just at the beginning of 

 discoveries along this line and it will no doubt be the work of the 

 physicians and scientists of the future to perfect many more ways 

 of producing immunity against protein poisons and germ disease. 



