I 



CHAPTER XXII 



ANTIGENS, HAPTENS, ANTIBODIES 

 AND COMPLEMENT 



N this chapter it is proposed to describe some of the 

 substances which enter into the reactions involved 

 in immunological phenomena. 



Antigens 



An antigen is a substance which, when introduced 

 parenterally into the animal body, calls forth the pro- 

 duction of another substance, known as an antibody, 

 capable of reacting specifically with the antigen. Anti- 

 gens always react with their corresponding antibodies, 

 but everything which reacts with an antibody is not 

 necessarily an antigen. In order to be antigenic it appears 

 that a substance must be (a) foreign to the animal into 

 which it is injected, (b) colloidal and (c) introduced beyond 

 the epithelial tissues of the animal. For our present 

 purpose we can divide antigens into those which occur 

 naturally and those which do not ; the latter we may 

 term artificial or synthetic antigens. 



Natural Antigens. — The natural antigens fall into 

 three classes, proteins, polysaccharides and lipoids. 



Proteins. — The vast majority of antigens are proteins 

 or contain a protein component. Almost all known 

 proteins are antigenic provided that they are soluble. 

 The notable exception is gelatin. It should be pointed 

 out that gelatin, strictly speaking, is not a naturally 

 occurring protein since it is produced by the hydrolysis 



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