IMMUNITY 81 



influence the immunity to a greater or less extent in 

 some instances. But it is just as probable that in some 

 cases the immunity is dependent upon conditions so 

 deep rooted and so unalterable as to defy environment, 

 food, or mode of living. From which we may gather 

 that in certain instances the natural immunity is only 

 relative, while in others it may be looked upon as abso- 

 lute. Unquestionably, many instances of natural im- 

 munity are closely related to acquired immunity in 

 their mechanism and cause, and are dependent upon 

 the presence in the tissues and body fluids of the sev- 

 eral complex and highly interesting substances which 

 are referred to collectively as antibodies and to the con- 

 ditions which their presence induces. We may, there- 

 fore, consider the manifestations of acquired immunity 

 for a better conception of the natural form of this con- 

 dition. 



ACQUIRED IMMUNITY 



Acquired immunity is said to be either active or pas- 

 sive. Active immunity is that form of immunity in the 

 production of which the cells of the individual take an 

 active part. Of this character is the immunity which 

 follows an attack of certain infectious diseases, as small- 

 pox. So also is that produced by inoculation with an 

 attenuated form of the disease, as in vaccination with 

 cow-pox, and that produced by injections of dead cul- 

 tures of the causative germ, i. e., bacterial vaccines. 



Passive immunity is defined as that form of immun- 



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