ANTIBODIES I 7 



any greater than before. Even recovery from the superficially very 

 similar disease German measles does not seem to confer any im- 

 munity to ordinary measles, and the child generally catches all the 

 common childhood infections, one after the other. If he misses one 

 or more of them he remains susceptible to it, as he demonstrates 

 by promptly coming down with it when exposed later in life, possibly 

 from one of his own children. However, this specificity is not ab- 

 solute. As an example we may mention that recovery from the rela- 

 tively mild flea-borne typhus caused by Rickettsia mooseri is fol- 

 lowed by immunity to the much more serious louse-borne typhus 

 caused by Rickettsia prozmceki (Rivers, 1952). 



Specificity of Antibodies 



Just as the immunity following recovery from infections is rela- 

 tively specific, so is the power of antibodies to react with antigens. 

 Diphtheria antitoxin will neutralize diphtheria toxin and possibly 

 save the life of a patient with diphtheria ; it does not neutralize 

 tetanus toxin and is of no value in the prevention or treatment of 

 tetanus. In general, antibodies seem to be adapted to react just with 

 the antigen . which called forth their production (homologous anti- 

 gen). But the specificity of antibodies, like the specificity of im- 

 munity, is not absolute. Antibodies produced by injecting rabbits 

 with purified ovalbumin from the hen react also with the ovalbumin 

 of various other birds such as the duck. A reaction of an antibody 

 with a related antigen is called a cross-reaction. It is generally not 

 as strong as the reaction of the antibody with the homologous anti- 

 gen. 



Today there is no doubt that the specificity of antibodies depends 

 on their chemical structure. But there is as yet no agreement whether 

 the specificity is a result of differences in amino acid composition or 

 in amino acid sequence, or merely of the way in which the polypeptide 

 chain is folded to produce the globular molecules shown in Fig. 1-3. 

 Pauling (1940) proposed the latter view. It is a fact that amino 

 acid analyses of antibodies have not yet revealed any clear-cut dif- 

 ferences in amino acid composition between different antibodies or 

 between particular antibodies and normal globulin (Boyd, 1956; 

 Smith et al., 1955). If the differences were mainly in amino acid 

 sequence and were confined to a "central, dififerential segment" 



