258 THE ANTIGEN-ANTIBODY REACTIONS 



of this chapter the capacity to act as a stimulant of antibody production was 

 included among its essential properties — it is, indeed, the property from which 

 its name is derived. The difference in behaviour between substances of varying 

 complexity, all bearing the same active specific grouping, led Landsteiner to 

 formulate the conception of the hapten, or partial antigen, which, while bearing 

 the specific grouping and therefore combining with the homologous antibody, 

 lacks certain of the properties of the complete antigen. 



In subsequent chapters frequent reference will be made to various polysaccharide 

 components derived from different types and species of bacteria. Many of these 

 polysaccharides, from the immunological point of view, occupy a position inter- 

 mediate between Landsteiner's simpler haptens and complete antigens. They react 

 specifically with the homologous antisera in vitro, giving precipitation ; but they 

 do not induce antibody formation in vivo, or, if they do so at all, the stimulus they 

 provide is of an altogether different order from that provided by the complete 

 antigen as it occurs in the bacterial cell. It seems likely that the range of activity 

 of a partial antigen is determined in the main by its molecular size. Landsteiner 

 and van der Scheer (19326) coupled succinic, adipic and suberic acids, through 

 their aniline compounds, to proteins, and prepared antisera against the com- 

 plete antigens so produced. Partial antigens were prepared by coupling the 

 same compounds to resorcinol. These partial antigens gave precipitation with 

 the corresponding antisera, and it was noted that the partial antigen derived from 

 suberic acid gave particularly strong precipitin reactions, probably on account of 

 its long aliphatic side chains. 



It is convenient to classify the antigenic complexes we have described under the 

 following headings : 



(a) Simple haptens, possessing immunological specificity, combining with the 

 homologous antibody and so preventing precipitation, but neither 

 forming a precipitate in vitro, nor stimulating antibody production 

 in vivo. 



(6) Complex haptens, possessing immunological specificity, combining with 

 the homologous antibody and forming a precipitate, but not stimulat- 

 ing antibody production in vivo. 



(c) Complete antigens, possessing immunological specificity, combining with 

 the homologous antibody and forming a precipitate, and stimulating 

 antibody production in vivo. 



It must not, however, be supposed that there is any sharp natural demarcation 

 between our named classes. It seems likely that the characters that determine 

 full antigenic potency are high molecular weight, associated with a large molecular 

 surface and a multiplicity of active specific groupings, and the relative activity of 

 these groupings themselves. If we were in a position to give a detailed description 

 of the chemical structure and immunological properties of a very large number of 

 antigenic substances, ranging from the simplest to the most complex, we should 

 probably find that they formed a continuous rather than a discontinuous series, 

 the falling-off in immunological activity as we passed from large and complex to 

 small and simple molecules being marked by the gradual loss first of one immuno- 

 logical property, then of another, until we were left only with the ability to combine 

 specifically with the antibody under optimal conditions of concentration and other 

 relevant factors. 



