CHAPTER III 



OUK KNOWLEDGE CONCEKNING NATUKAL IMMU- 

 NITY, ACQUIKED IMMUNITY, AND AKTIFICIAL 

 IMMUNIZATION 



NATURAL RESISTANCE AGAINST INFECTION 



IN the preceding chapters we have confined ourselves largely to 

 the consideration of those properties of the bacteria which determine 

 their ability to infect. In this discussion, however, we have repeat- 

 edly emphasized the fact that every infectious disease is the result of 

 a struggle between two variable factors the pathogenic powers of 

 the bacteria on the one hand, and the resistance of the subject on the 

 other, each of these again modified by variations in the conditions 

 under which the struggle takes place. Thus a given micro-organism 

 may be capable of causing fatal infection in one individual but may 

 be only moderately virulent or even entirely innocuous for another. 

 Conversely the same individual may be highly susceptible to one va- 

 riety of bacteria, but highly resistant to others. Even in reactions 

 with one and the same micro-organism, the susceptibility or resist- 

 ance of the individual may be determined by variations in the physi- 

 ological state or by the environmental conditions under which the 

 two factors invader and invaded are brought together. There- 

 fore, the conceptions "resistance," "immunity," and its opposite 

 "susceptibility," are relative terms which can never be properly dis- 

 cussed without careful consideration of all modifying conditions 

 which influence them. 



The science of immunity deals with a detailed analysis of these 

 variables. Its ultimate practical aim is the determination of meth- 

 ods by which an original susceptibility can be transformed into re- 

 sistance or even immunity. And the rational method of approach- 

 ing this subject consists in a careful study of the conditions of sus- 

 ceptibility and immunity as they exist naturally in the animal king- 

 dom. 



The mere fact that both animals and man are in constant con- 

 tact with infectious micro-organisms, many of them in a high state 

 of virulence, indicates in itself that the animal disposes normally 

 over a defensive mechanism of considerable efficiency. 



To a certain extent, of course, this escape from harm is due to 



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