IMMUNE SERA 



I. INFECTION AND IMMUNITY 



Infection. An infectious disease is one caused 

 by a living organism which 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 organisms are con- 

 cerned, we know that they may remain localized 

 or be widespread through the body. The absorp- 

 tion 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 



