51 



and, in the course of metabolism, particularly in the living body, 

 pass through a large variety of changes, including chemical 

 grouping and re-grouping, which are dependent on the conditions 

 of metabolism. New groups of antigenic value (i.e., new antigens 

 or new antigenic components) may emerge at any stage of these 

 processes, just as some of the old groups (i.e., the original antigen) 

 may be lost. 



This conception of chemical instability in the antigen, which 

 I think must be accepted as reasonable, has an important bearing 

 on one's ideas of " antigenic components." In one important 

 respect it does not supersede the idea, current amongst bacterio- 

 logists, of multiple antigenic components existing side by side. 

 In the preceding paragraphs, examples have been given which 

 illustrate the elements of truth in that view. But, in another 

 and perhaps more important respect, the idea of the evolution 

 and devolution of antigenic properties as a normal event of 

 metabolism is incompatible with a strict interpretation of the 

 " mosaic pattern " hypothesis, according to which every demon- 

 strable antigenic function is referable to a special chemical group, 

 preformed in the original molecule. The difference between 

 the two views may be expressed briefly. According to the 

 former, an antigenic function is referable to a chemical group b, 

 which may be either (1) pre-existent in the original molecule and 

 in an active state therein, or (2) pre-existent in the original 

 molecule but in a masked condition and only able to manifest 

 its activity if subsequent changes remove inhibitory conditions, 

 or (3) non-existent in the original molecule and newly created 

 in the course of subsequent changes. According to the latter 

 view, b must be either (1) or (2). 



Possible examples of (3) would be : — (a) production of agglu- 

 tinins by immunisation with an inagglutinable strain of bacteria, 

 or (b) production of antitoxic sera by immunisation with bacteria 

 which, in the intact condition, are non-toxic, or (c) production 

 of sera which are antibacterial in vivo by immunisation with 

 bacteria towards which the sera are not antibacterial in vitro, or 

 (d) modification of antigenic character by tryptic digestion. 



But thje alternative view, which would eliminate (3), cannot 

 be disregarded. It raises wider questions, which will now be 

 considered more fully, about the value of Durham's hypothesis 

 as an explanation of immunity. 



The " Mosaic Pattern " Theory. 



According to this doctrine, antigenic characters consist of 

 certain preformed chemical groups which are present in the 

 protein molecules. There may be several of these groups. They 

 are often imagined as forming a mosaic pattern, the complexity 

 of which corresponds to the range or complexity of the antigen. 

 In the production of an immune serum, each unit of the antigen 

 mosaic gives rise to a corresponding or counterpart group in 



