ANTIGEN-ANTIBODY REACTIONS 455 



beginning of the inhibition zone it corresponds to S4A, and 

 the soluble complex is S5A. With Type III horse antisera 

 the compounds are S3 A to SeA for the equivalence zone 

 and SioA for the soluble complex in the inhibition 

 zone. 



The combination of antigen and antibody takes place 

 as though the molecules behaved as fairly rigid bodies, 

 the antigens being roughly spherical in shape and anti- 

 bodies more or less ellipitical, with a ratio of length to 

 breadth of about 7 to 10 (see page 429). Antigen molecules 

 are probably multivalent in that they contain several 

 reacting sites or determinant groups, even when the 

 determinant groups are all the same. They will usually 

 have a higher valency the larger the molecule. If Pauling's 

 conception of antibody formation is correct antibody 

 molecules are for the most part to be regarded as divalent, 

 although some monovalent molecules are also formed 

 (see page 423). If the antigen and antibody are of 

 about the same size then at the equivalence point the 

 ratio of the weight of antibody to the weight of antigen 

 in the precipitate would be about A^/2 where N repre- 

 sents the valence of the antigen and the limiting values 

 would be 1 for excess of antigen and N-l for excess of 

 antibody. When the antigen is much larger than the 

 antibody the ratio would be less than N/2 at the equiva- 

 lence point. The value of N, the effective valence of an 

 antigen, is determined by the number of antibody 

 molecules which can be packed round the antigen molecule. 

 If the two molecules are spheres of equal size, 12 antibody 

 molecules can be fitted into place round one antigen 

 molecule. If the antigen is larger than the antibody 

 more molecules of the latter can come into contact with 

 the antigen and N may be larger than 12 ; if the antigen 

 is the smallei' molecule then N is less than 12. These 

 expectations are borne out by experiment, as the values 

 of A^ in Table 34 show :— 



