212 Bernard N. Jaroslow and Henry Quastler 



II. NOMENCLATURE AND MODEL 



An antigen, GN, consists of a specific detenninant G and a carrier moiety 

 A'^ which is a macromolecule (MW > 10,000), i.e. protein, hpo-protein, glyco- 

 protein, etc. G may consist of three or four amino acid residues, a mono- or 

 disaccharide, an aliphatic chain, etc., with its specificity dependent upon order, 

 size, polarity, or optical configuration of the residues. There may be a number 

 of antigenic determinants, of the same or different specificities, on the surface 

 of N, so that one molecule of antigen may combine with several molecules 

 of antibody, e.g. 5 for ovalbumin to over 200 for hemocyanins (3). Combining 

 capacity as well as antigenicity (ability to induce antibody formation) is usually 

 proportional to the molecular weight of A'^. 



An antibody, AB, consists of a specific combining site A and a carrier 

 moiety B which is always a globulin, usually y-globulin. The combining site 

 is believed to be a chemical and/or spatial configuration which combines with 

 the specific antigenic determinant through hydrogen bonds. The number of 

 combining sites per antibody molecule is thought to be two or one. 



The A-G reaction may be manifested in a number of ways: precipitation 

 of a soluble antigen, agglutination of a particulate or cellular antigen, or lysis 

 of a cellular antigen in the presence of complement. 



We consider a heterophile reaction as one in which the reaction between 

 C2 and y^i is indistinguishable from the homologous reaction of G^ and A-^ 

 although G2 and G^ come from different sources. By cross reaction we mean 

 the phenomenon wherein G2 reacts with A^ but the strength of reaction is less 

 than that of the homologous one {G^ and A^. 



We can describe an antigenic determinant, G, as a 'word' of k 'letters'. 

 By letter we mean antigenically active residues such as amino acids, mono- 

 saccharides, etc. Let r be the size of the alphabet, i.e. the number of available 

 letters; then 



H (letter) = a • logg r. 



a is a constant which ranges from zero to one. Its upper limit occurs if all 

 'letters' occur with equal probabihties, and no two letters can ever have equiva- 

 lent effects. 



The average information content of a worfi? averaging k letters is given by: 



H(G) = ^- k- H (letter). 



/9 is a constant which ranges from zero to one. Its upper limit occurs when 

 all letter combinations are equally probable, i.e. if there is no 'intersymbol 

 influence'. The lower limit would obtain if there existed only one antigenically 

 active combination of letters. 



To fix the ideas on the measuring of /-, H (letter), k and H(G), we give 

 the corresponding values for printed English: 



r = 26 k {=ii4.5 



log2 r = 4.7 ^ f^ 0.6 



a ^ 0.87 H{G) ^ 10 



//(letter) = 4.1 



