THE FACTS OF IMMUNITY 



eluding antibody production, occur (a) if antigens carrying 

 foreign determinants enter the body, [b) if body components 

 normally inaccessible become 'accessible' for any reason, 

 or {c) if antibody-producing cells deviate from the limita- 

 tions implanted during embryonic life. 



It may be desirable to elaborate this interpretation slightly 

 in the light of what was said earlier about antigenic deter- 

 minants. Figure 5 is a highly schematic representation of an 

 antigenic protein molecule. Antigenicity must of course 

 always be relative to the host — the animal being used for 

 antibody production — and the surface of every protein anti- 

 gen will carry hundreds of potential antigenic determinants 

 which are inert because they are the same as determinants 

 already present in host components. The antigenic quality 

 of the molecule depends on the presence of a small number of 

 antigenic determinants A, B, etc. which differ from any 

 present in host components. A diagram of this type will 

 perhaps serve to indicate the magnitude of the ' information ' 

 problem involved in differentiating self from not-self. 



4. The qualitative types of immune response 



The subject-matter of immunology has often been un- 

 consciously confined to the high-titre antibodies produced 

 by the immunization of horse or rabbit with diphtheria toxin 

 or some other of the classical antigens. Such antisera react 

 with the antigen by aggregation in the test tube and by 

 * neutralization ' of the biological function of the antigen 

 when this can be tested. Most of the practical applications 

 of serology make use of such antisera and all the classical 

 academic work in immunology is based on their properties. 

 'Classical circulating antibody' is, however, far from being 

 the only product of specific response by the body to antigenic 

 material and it is, possibly, not even the most important. 



Perhaps the simplest way to indicate the differences that 

 may exist in qualitative type of immune response is to 

 compare different types of response to the same antigen, 



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