IMMUNITY. 13 



or modified course of the disease. Acquired immunity may be 

 either active or passive. 



Experimental active immunization is usually called vacci- 

 nation and generally produces in the individual a modified form 

 of the disease. All, or nearly all, of the symptoms are less severe 

 than in the natural course of the disease. The individual in this 

 case produces his own immunity. In artificial or intentional in- 

 oculation, the etiological factors, or more particularly, the causal 

 organisms injected must be so modified that the natural course 

 of the disease will not follow the inoculation or injection. Ex- 

 perimentally acquired active immunity is produced by the in- 

 jection of living or killed microorganisms, or of toxins produced 

 by these organisms. When living organisms are injected their 

 virulence is usually reduced by passage through animals, growth 

 at high temperatures, on artifical media, in the presence of chem- 

 icals, or growth in the presence or absence of oxygen, etc. Active 

 immunity may also be acquired as a result of injection of living 

 virulent organisms into such parts of the body where the disease 

 will not be produced. The first amounts injected are usually 

 smaller than those in subsequent injections. When bacteria are 

 injected the immunity produced is a bacterial immunity and 

 when toxins are injected a toxin immunity results. 



In passively acquired immunity, the individual that becomes 

 immune does little or nothing toward obtaining this immunity. 

 It results from the injection of immunizing substances which 

 have been prepared by an actively immunized individual or 

 animal. 



There are two classes of immunizing substances; those 

 that act on bacteria, said to be anti-bacterial; and those that 

 act on toxins, called anti-toxic. Of these two classes of immune 

 substances, the anti -toxic has been more efficient in passive im- 

 munization. 



After the formation of immunizing substances they do not 

 remain indefinitely within the body, but are lost through the ex- 

 cretions either as immune bodies or as immune bodies broken 

 down by the body cells. On this the difference in persistance of 

 natural and acquired immunity is partly based, for in acquired 

 immunity the supply of immune bodies is exhausted, while in 

 natural immunity new substancss are constantly formed. 



