CHAPTER 3 

 Antigens 



Definition 



We use the term antigen in at least two senses. Primarily, an 

 antigen is a substance which, when introduced into an animal, usually 

 not by way of the digestive tract, causes the production of specific 

 antibodies. Immunologists commonly use the term antigen also for 

 preparations which merely react with antibodies in vitro ; for in- 

 stance, the mixture of normal tissue lipids used in the Wassermann 

 test for syphilis is referred to as the Wassermann antigen, although 

 injection of it into an animal would probably not cause the produc- 

 tion of syphilitic antibodies. 



Immunologists are also imprecise in another way in their usage 

 of the term antigen. The word is applied to both a purified, supposedly 

 molecularly homogeneous preparation, as for example crystalline 

 bovine serum albumin, which, when injected, will cause the pro- 

 duction of antibodies to this substance, and also, following tradition, 

 to preparations which, chemically speaking, are complex mixtures, 

 such as the suspensions of killed organisms which are used in the 

 practical production of certain types of immunity. 



Antigenicity 



In spite of many studies on the subject we are not yet in a position 

 to state positively what physical and chemical characteristics make a 

 substance antigenic. We can only offer certain rules of thumb: (a) 

 antigens are substances the molecules of which are larger than mini- 



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