210 THE ANTIGEN-ANTIBODY REACTIONS 



hydrophile groups remain to slow, or totally to inhibit, precipitation. The com- 

 pounds are schematically represented in Fig. 33, where D, E and F represent 

 an antigen molecule after combination in the zones of antibody excess, equivalence 

 and antigen excess. 



The compounds formed according to Marrack's lattice hypothesis are also illus- 

 trated in Fig. 33 A-C, which represent two-dimensionally the reaction of a hypo- 

 thetical hexavalent antigen and trivalent antibody. In gross antigen excess, all 

 three antibody valencies are occupied by antigen ; the solution consists of quadri- 

 molecular complexes which cannot combine with one another to form larger com- 

 plexes because only antigen valencies remain unsatisfied. In a lesser antigen 

 excess, complexes as large as A^ are formed ; but these cannot aggregate more 

 fully since again only antigen valencies are unsatisfied. B and Bi illustrate simi- 

 larly two complexes formed in antibody excess, and C represents an early stage 

 in lattice formation at the equivalence point ; there are free antigen and antibody 

 valencies for the extension of the lattice indefinitely. 



On this view the hydrophobe character of the larger aggregates is ascribed 

 not only to a loss of attraction for water, but also to a specific attraction between 

 the particles or molecules of which the aggregates are composed. This conception, 

 as Marrack points out, is entirely compatible with the dependence of flocculation 

 on the presence of salt in moderate concentration. The action of the electrolyte, 

 in reducing the negative charge on the molecules, or particles, will assist aggrega- 

 tion, whether this be due to a loss of attraction for water or to mutual attraction 

 between one molecule, or particle, and another. 



Similar arguments apply to the constant-antigen titration. As antibody con- 

 centration increases, the antigenic particles are more and more rapidly and fully 

 sensitized, and the speed of flocculation increases. In antibody excess, the 

 complexes formed are more soluble than those at the optimum. The marked 

 inhibitory effect of excess antigen, however, in the constant-antibody titration 

 is not usually paralleled in the constant-antigen titration by inhibition in antibody 

 excess. As we have seen (p. 203), the addition of antibody in excess of constant- 

 antigen equivalence produces even more rapid flocculation, so that the constant- 

 antigen optimal ratio of antigen to antibody is higher than the equivalent ratio 

 (see also Duncan 1937). Marrack (1934) suggested that if antibody but not antigen 

 were denatured during the combination of the two, compounds like A (Fig. 33) 

 would be soluble, since the antibody is protected by the fully hydrophile antigen, 

 whereas compounds like B would be hydrophobe, since partial denaturation of 

 the antibody at the surface would take place. Moreover, since it appears that 

 antigen has a greater valency than antibody, in antibody excess there will be a 

 greater packing of antibody round the antigen than of antigen round antibody 

 in antigen excess. We have already noticed that in constant-antigen titrations 

 of systems like diphtheria toxin and horse antitoxin antibody excess has a sharply 

 inhibiting effect. Thus precipitation may be completely inhibited by doubling 

 the concentration of antibody that gives optimal precipitation (Healey and Pinfield 

 1935, Pappenheimer and Robinson 1937). The work of Pappenheimer (1940) 

 suggests that the narrow flocculating zone, sharp inhibition by antibody excess, 

 and the easily elicited Danysz phenomenon of horse antitoxic sera are dependent 

 on the nature of the antibody. Horse antisera to diphtheria toxin and ovalbumin 

 exhibited all three properties, whereas the corresponding rabbit antisera to the 

 two antigens did not. Boyd (1941) came to a similar conclusion from a study 



