IX. INFECTION AND IMMUNITY 



Infection. In the preceding chapters we have 

 studied the formation and mode of action of the 

 various antibodies. Let us now summarize briefly 

 our knowledge concerning the factors involved 

 in infection and immunity. An infectious disease 

 is one caused by a living organism wiiich has gained 

 access to the tissues of the body. A study of 

 infection and immunity, therefore, embraces a 

 study of the pathogenesis of these organisms on the 

 one hand and of the defensive agencies of the body 

 on the other. So far as the invading organism 

 is concerned, we know that this may remain localized 

 or be widespread through the body. The absorption 

 of chemical products from a local infection may 

 produce general symptoms. This is known as an 

 intoxication, and is observed in cholera, diphtheria, 

 tetanus, local abscess, etc. In general we apply 

 the term ' pathogenic ' to organisms capable of 

 producing disease, but it must be borne in mind 

 that this is a relative term, for an organism patho- 

 genic for one species of animal need not necessarily 

 be pathogenic for another species. 



The Infecting Agent. In studying the patho- 

 genicity of various bacteria, it is apparent that 



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