IMMUNITY AND ITS VARIETIES. 99 



1. Active acquired immunity is produced by the action of 

 living germs or their toxins introduced into the animal. 



2. Passive acquired immunity is obtained by a direct trans- 

 ference of an immunizing substance from an immune animal 

 to a susceptible one. Active immunity takes some time to 

 develop, but, as a rule, lasts longer than passive immunity, 

 which is immediately established. 



Methods of Producing Immunity. 

 A. Active Immunity. 



Active immunity is produced as follows by : 



I. Inoculation, or the introduction of small quantities of live 

 bacteria, so as to produce a mild attack of the disease. This 

 method is dangerous from the fact that it is hard to ascertain 

 how small a quantity of bacteria may be introduced without 

 its being prejudicial to life, and from the danger of spreading 

 the infection. 



II. Vaccination, or the introduction of attenuated bacteria, 

 which attenuation is obtained either by submitting the bac- 

 teria to a higher degree of heat during their cultivation or by 

 adding a small proportion of an antiseptic to their culture- 

 media, or by using bacteria which have grown for a long 

 period of time in artificial media, or by using bacteria which 

 have grown in the bodies of natural immunes. 



Vaccination with dead bacteria, first suggested by Chauveau 

 and elaborated by Wright and others, is a method of active im- 

 munization in much favor. This method has proved very useful 

 in antityphoid vaccination, and in a lesser degree in plague and 

 Asiatic cholera. 



III. Intoxication, or the introduction of the toxins of the 

 bacteria in small broken, but frequently repeated doses, or in 

 cases where the toxic effect of bacteria is due to substances 

 contained in the cell-body itself by the injection of the dead 

 bacilli. This is the method used for the production of the 

 diphtheria and tetanus antitoxins. 



IV. By the inoculation of an emulsion of tissues, consisting 

 in the introduction into the animal of the emulsion of certain 



