ANTIGENIC STRUCTURE IN RELATION TO REACTIONS 285 



At E is a rough variant from D. The capsule has gone, and with it the char- 

 acteristic capsular antigen. The character of the cell surface is now determined by 

 the antigen that before was hidden. 



All these diagrams, and particularly D and E, are much too simple to be true. 

 There are probably a very large number of antigens, or antigenic groupings, in 

 any bacterial cell. But it is true that the character of the cell surface appears in 

 most cases to be dominated by a few characteristic antigenic components, and that 

 this surface may be profoundly altered by the replacement of these components 

 by a few others. It is probable that the S — >■ E, variation, at least as it manifests 

 itself in a culture of bacteria, is not a sudden, all-or-none mutation, but a more 

 gradual or step-like process, with intermediate forms in which the normal surface 

 antigens are still present, though in diminished amount (see, for instance, Wilson 

 1930). 



Antigenic Stricture in Relation to the Different Antigen- Antibody Reactions. 



When antibodies unite with antigens that form part of a bacterial cell, the 

 effect on the cell will vary according to the position that the antigen occupies. 



If the antigen is situated on the surface of the flagella the antigen-antibody 

 compound will be formed in this situation, the usual change in the direction of 

 salt- sensitiveness will occur, and, in the presence of electrolytes, the bacilli will 

 flocculate in the large, loose clumps characteristic of flagellar agglutination. 



Antigen-antibody union in this situation does not, however, render the bacterium 

 sensitive to lysis by complement, and it is relatively ineffective in promoting 

 phagocytosis (see Felix 1924, Braun and Nodake 1924, Felix and Robertson 1928). 

 For sensitization of this type to occur antigen-antibody union must take place at 

 the bacterial surface. 



It would seem, also, that the aggregates formed in flagellar agglutination are 

 not of the kind that readily fix complement ; though there is some uncertainty on 

 this point (see Hofmeier 1927, Springut 1927). It seems possible that the absence, 

 or paucity, of complement fixation may be due simply to the physical character 

 of the aggregates and the rapidity with which they are formed. 



The more deeply seated antigens will play no part in the serological reactions 

 of the intact cell in its normal smooth form. They are inaccessible to the antibody. 

 In suspensions that have been allowed to autolyse, or that have been broken up 

 in some other way, these antigens may be liberated. They will then unite with the 

 corresponding antibody to form a precipitate, and if complement is present and the 

 conditions favourable this complement will be fixed. 



To revert for a moment to the problem of the unity or diversity of antibodies, 

 it is logical to say that an antibody acting on a flagellar antigen is an agglutinin, 

 but not a lysin ; that an antibody acting on a surface antigen is an agglutinin and 

 a lysin, and a complement-fixing antibody, and an opsonin ; and that an antibody 

 acting on an antigen that is normally situated below the bacterial surface is, when 

 tested against bacterial extracts or autolysates, a precipitin and a complement-fixing 

 antibody, but is not, so far as the normal cell is concerned, an agglutinin, or a lysin 

 or a complement-fixing antibody or an opsonin. But none of these differences 

 in reaction are determined by differences in the nature of the antibody, apart from 

 its combining group ; they depend on differences in the situation of the antigen 

 with which it unites. 



