52 



the serum. These counterparts, attached to certain elements in 

 the serum, form the complex known as antibody. The antigen- 

 antibody reaction consists in the union, subject to favourable 

 colloidal conditions, of preformed antigen groups with their 

 preformed counterparts or affinities which are present in the 

 antibody. This is a development of the well-known " lock and 

 key " conception of Emil Fischer, which has proved of great 

 value in many respects. 



Difficulties arise at once in the application of this conception 

 to known facts. Normal sera react with a very large variety 

 of substances in a way which cannot be distinguished from an 

 antigen-antibody reaction. It must therefore be postulated 

 that these non-specific reactions, also, are due to structural 

 groups, each " fitting " exactly with a corresponding group in 

 any antigen with which it happens to react. So the mosaic 

 pattern of an immune serum must be sufficiently complex to 

 include its " mosaic " as a normal serum along with its special 

 " mosaic " which it has acquired by immunisation. 



There is a further complication in the pattern. In the 

 production of antibodies, non-specific factors frequently play an 

 important part. For example, the agglutinin produced by 

 immunising a rabbit with a particular bacillus may be due not 

 merely to the specific antigenic group but to the complex of 

 specific and non-specific elements contained in the inoculum. 

 The reason for this supposition is the observation that a rise in 

 titre for the same strain is often obtained by subsequent inocula- 

 tion with a bacillus of quite different species. Or, again, it has 

 been alleged that protection against a particular bacterium may 

 sometimes be obtained by vaccination with an entirely different 

 virus, or, possibly, by using a vaccine which is not prepared from 

 any bacterial protein. In general, there are many indications 

 that great importance should be attached to this " non-specific " 

 acquired immunity.* 



According to the " mosaic " conception, all the above facts 

 and difficulties are to be accounted for by postulating that a 

 bacterial antigen consists of an indefinitely large number of 

 parts, a, b, c, d, &c, and that the specificity resides in the com- 

 plex as a whole, though certain of the components also occur 

 in the antigens of other bacterial species or even in non-bacterial 

 protein. This overlapping of a or b, &c, and the overlapping 

 of the corresponding constituents in the antibodies would account 

 for non-specific influences. In other words, the non-specific 

 element is to be explained by an indefinite multiplication of the 

 units which are supposed to explain specific reactions. 



In discussing the value of the postulate which I have outlined 

 in the preceding paragraphs, one must first note that it is 

 biological rather than biochemical, in the sense that it is derived 



* Here I am assuming, provisionally, that the antigen-antibody con- 

 ception suffices to explain immunity. In a later section (pp. 59-63) I 

 suggest that this assumption may not always be valid. 



