ANTIGENS, HAPTENS, ANTIBODIES, ETC. 423 



molecules are almost constantly associated Avith already 

 modified ends of antibody globulin chains and therefore 

 not free to influence the production of further antibody 

 molecules . 



This theory accounts for the fact that more than one 

 sort of antibody can be produced by a single antigen if 

 that antigen carries more than one determinant group 

 which can act as a template for the coiling of ends of the 

 peptide chains. Thus it is known that the heat stable 

 antigen and the heat labile antigen of the vaccinia virus 

 are two parts of a single antigen molecule. That is, 

 antigens may be monovalent or, more usually, since they 

 are complex molecules, multivalent, in the sense of being 

 able to combine with one or more antibodies. Thus if an 

 antigen had two hapten groups, A and B, it would give 

 rise to divalent antibodies carrying the specific end 

 groups A' — A', B' — B' and A' — B' as well as monovalent 

 antibodies, A' and B'. The fact that antibodies formed 

 as a result of prolonged immunisation usually have a 

 broader specificity and give wider equivalence zones of 

 reaction with antigens (see Chapter XXIII) is also 

 accounted for on this basis. The theory also accounts 

 for the cross reactions which occur between the antisera 

 to different antigens since several antigens may contain 

 a common determinant group. The apparent paradox 

 that antigens with powerful polar determinant groups 

 are frequently poor antigens, giving only low concentra- 

 tions of antibody although of sharp specificity, whilst 

 antigens with weak polar groups, that is of low specificity, 

 usually give high titre antisera is explained by the fact 

 that the stronger the polar group the more firmly will 

 it attract and hold the complementary antibody group 

 and therefore the less frequently is the antigen molecule 

 available to influence the formation of fresh antibody 

 molecules . 



On the basis of this theory Pauling predicted that if 

 normal glolnilin Avere j^laced under mild denaturing 



28 



