36 INTRODUCTION TO IMMUNOCHEMICAL SPECIFICITY 



other hand, Gill and Doty (1960) found a synthetic linear polymer 

 containing tyrosine, which would increase the rigidity of the mole- 

 cule, to be antigenic, and Sela (1962) found that a multichain 

 copolymer, in which the chains of polypeptides containing L-tyrosine 

 and L-glutamic acid were built on a multichain poly-DL-alanine, is 

 a powerful and sharply specific antigen. 



It was once believed that only proteins could be antigenic, but 

 we now know that some carbohydrates are also good antigens. The 

 rigid sites in polysaccharide antigens may be the pyranose or 

 furanose rings (Sela, 1962). Large-molecule carbohydrates vary 

 in their antigenicity. Pneumococcus polysaccharides are antigenic in 

 man and in the mouse but not in rabbits (Dubos, 1945). Dextrans, 

 apparently not antigenic for rabbits, are antigenic in man (Kabat 

 and Berg, 1952, 1953). Purified blood group substances A and B 

 are fair antigens in man but not in rabbits (Morgan and van Heynin- 

 gen, 1944; Kabat, Baer, Day, and Knaub, 1950). 



We repeat that substances must be foreign to the circulation to 

 be antigenic for an animal. The normal animal does not produce anti- 

 bodies to the protein and carbohydrate constituents of its own blood 

 or to the tissue components which ordinarily reach the blood. Normal 

 animals, however, can be induced to form antibodies to constituents 

 of their bodies which normally do not find their way into the cir- 

 culation, such as lens protein of the eye and casein, even from an 

 animal's own milk (Lewis, 1934). 



At one time it was believed that only proteins could be antigenic. 

 We now know that many carbohydrates are also good antigens. Other 

 classes of antigens exist, but with some possible exceptions all of 

 them contain some protein or carbohydrate, or both. 



Immunological Tolerance 



For a long time it was a complete mystery why an animal did not 

 make antibodies for the proteins and other substances of his own cir- 

 culation, many of which are good antigens for an individual of a 

 different species. A clue has recently been found in the phenomenon 

 of immunological tolerance. If embryos are injected in utcro or early 

 in postnatal life with an antigen, not only may they not produce 

 any antiboch' to the antigen, but they may be rendered incapable of 

 responding to this antigen for the rest of their life (Burnet, 1956) 



