48 



Chemical Specificity. 

 One must first try and form some mental picture of what 

 is meant by specificity; and it will be convenient to take the 

 chemical aspect before going on to discuss colloidal conditions. 

 The balance of opinion is in favour of the view that all 

 substances which act as antigens contain large and highly complex 

 molecules and are of a protein nature, or at least cannot be 

 dissociated from protein.* Pick believes that a further chemical 

 characteristic of an antigen is the possession of an aromatic 

 nucleus. He regards this aromatic complex as the central ring 

 for the grouping of the side-chains, upon the arrangements of 

 which specificity depends, since the aromatic nucleus, as such, 

 cannot possibly account for the enormous number of possible 

 variations which nature affords. This wide range of variation 

 must depend upon differences in the position of different groups, 

 or side-chains, within the molecule, including differences in 

 stereo-chemical configuration. 



Remembering that the above conception is to be supplemented, 

 later on, by a consideration of colloidal conditions, and that it 

 by no means implies acceptance of what is known as Ehrlich's 

 theory of immunity, I think it may be regarded as a non- 

 controversial way of forming a starting point in the attempt to 

 arrive at a rough chemical representation of the specific property 

 of an antigen. 



The biochemists have provided some interesting data which 

 enable one to fill in the picture with certain details illustrative 

 of antigenic characteristics. 



1. The " dominant antigen." — If a protein (the serum of an 

 animal) is convertedinto a nitro-protein by the action of nitric acid, 

 a profound change in antigenic specificity is produced. The 

 original specificity (i.e., the specificity characteristic of the animal 

 species) is lost and is replaced by a new specificity which is common 

 to all nitro-proteins and is peculiar to these compounds. For 

 example, nitrified rabbit serum, when used as an antigen, will 

 produce an antiserum (either in rabbits or in other species of 

 animals) which will precipitate the nitro-protein obtained by 

 nitrifying the protein of any species of animal but will not give 

 a characteristic reaction with normal rabbit serum. Similar 

 changes in specificity are produced by converting serum proteins 

 into iodised proteins or into diazo-proteins ; the original serum 

 specificity is lost and a new specificity common to all iodo- 

 proteins, or to all diazo-proteins, is acquired. 



A particular chemical factor, therefore, may completely 

 dominate antigenic specificity. Perhaps a factor of this nature 

 is dominant in certain bacteriological species, the members of 

 which appear to be serologically uniform; e.g., it appears to be 

 a general rule that all true vibrios of Asiatic cholera should 

 agglutinate with the same stand ard serum. 



* I think confirmation is needed for the suggestion that protein-free 

 lipoids may act as antigens. 



