49 



2. Limitation of specific reaction due to multiple antigenic 

 components. — Chemical influences of a less drastic nature than 

 the above may alter the original specificity in another way, not 

 by abolishing it, but by adding to the original antigen a new 

 antigenic component which makes the modified antigen less 

 wide in its range of action than the original antigen. To quote an 

 example given by Pick, an antiserum prepared by immunising with 

 diazobenzol-ox -protein will precipitate only diazobenzol-ox -protein, 

 not normal ox-protein and not the protein of man, horse, or dog, 

 which has been turned into the diazobenzol compound. In 

 general terms, when an antigen A (here the serum of a particular 

 species of animal) is linked to a new antigenic character, b (here 

 a known chemical compound), the combination A6 may retain 

 its original specificity (due to A) whilst acquiring a new " con- 

 stitutive " specificity (due to b) ; thus the antiserum produced 

 by immunising with A6 may react only to A6 antigen, not to 

 6 coupled with an antigen other than A, not to the normal A 

 antigen, and not to A coupled with c or d or e, &c. 



One cannot fail to note the resemblance, which may or may 

 not be of intrinsic importance, between these experimentally 

 modified antigens and the idiosyncrasies of certain normal 

 bacterial antigens. To take the readiest example, one may 

 suppose that all pneumococci possess, as such, a common anti- 

 genic nucleus (N) and that racial differences are due to the 

 possession, in addition, of special antigenic properties (6, c, d, &c.) ; 

 it is found, however, in agglutinin and precipitin tests, that N6 

 antiserum does not react with Nc or Nd antigen, but only with 



m. 



3. Increased breadth of reaction due to multiple antigenic 

 components. — It has already been mentioned that a native serum 

 protein (A), the nitro-protein (B) of the same serum, and the 

 diazo-protein (C) of that serum behave as three entirely distinct 

 antigens. But, to quote an example given by Pick, if A is 

 linked to 6 (diazobenzol), and B is linked to 6, and C is linked to 

 both 6 and c (a-naphthol), it is found that the three antisera 

 produced by immunising with (1) A6, (2) B6, and (3) Cbc all 

 react equally well with A6 antigen. Here is an example which 

 is the converse of the last. A, B and Cc behave as antigens 

 having nothing in common, but the addition to each of the 

 component 6 brings out a very striking inter-relationship between 

 the three antigens thus formed. 



This result provides an apparently close parallel to what 

 is very frequently found by bacteriologists. Studies of the 

 serological races of meningococci and of the Flexner group of 

 dysentery bacilli are good examples. Strains are found which 

 behave with apparent inconsistency when different serological 

 criteria are applied to them, being distinct from each other 

 according to some tests and closely akin when other tests are 

 applied. The natural explanation of the resemblances between 

 strains which also differ from each other is suggested by the 



