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B or 6, &c. ; for example, A6 antigen will react with any serum 

 containing either A or b antibody. 



(4) There remains a fourth possibility. A group of organisms 

 may come under category (3) in some respects but with the 

 exception that, in certain instances, there is a reversion to con- 

 dition (2) ; e.g., the union of A with a chemical group d may 

 mask the antigenic properties of A. 



In discussing bacterial antigens it is often assumed that, if 

 the structure of the bacterial cell were known in every chemical 

 detail, this information would provide a complete record of 

 antigenic properties, at least in their chemical aspect. I do not 

 think that this assumption is correct. Antigen is a relative 

 term, meaning capacity to produce antibodies in the living animal. 

 When the bacterial protoplasm becomes modified by interactions 

 taking place in the animal body, it seems to me probable that 

 new antigenic capacities may emerge, and that these may depend 

 upon peculiarities of chemical structure which were not represented 

 in the molecules of the bacterial culture used for inoculation. 



This view is in some respects incompatible with the " mosaic 

 pattern " theory. I think this theory has been pushed a great 

 deal too far as an attempted explanation of the nature of anti- 

 bodies and their relations to antigens. Up to a certain point, 

 the conception of multiple antigenic components is useful and is 

 substantiated by experiment. It has been shown that some 

 bacterial antigens are complex, and the fact that they consist 

 of different antigenic components can be demonstrated by the 

 selective action of the complex antibodies which they produce. 

 So far the ground is safe. But, when one begins to represent this 

 complexity in diagrammatic fashion by means of measured 

 lengths and areas (showing that the antigen of a certain strain 

 consists of \a-\-\b-\- £c, and so forth), one must beware of pitfalls, 

 since it is very doubtful if such diagrams give any idea of the com- 

 plicated chemico-physical reactions which really take place. 

 And this is only the beginning of the danger. For a simple 

 jDroblem in serological classification three or four units may 

 suffice to piece together all the different " mosaic " diagrams 

 which it is desired to postulate ; but complicated problems in 

 immunity will require the introduction of a great many more 

 units. If the diagrams are accepted in the former case, they ought 

 to be accepted in the latter. And so one becomes confronted with 

 mosaic patterns of ever increasing complexity. This, it appears 

 to me, is the logical outcome of the theory. It leads to con- 

 clusions which I regard as unsubstantiated by fact and unsound in 

 principle. 



According to the biochemists, a good antigen should possess 

 molecules which are not rapidly broken up in the animal body; 

 otherwise, the antigen may disappear before it has persisted 

 long enough to stimulate antibody production. I am not aware 

 of any experiments showing that amongst bacterial antigens 

 there are important differences in "staying power"; but I 

 think this possibility is worth bearing in mind. 



