66 



antigenic " mosaic." These are matters of importance, parti- 

 cularly in view of the efforts which are now being made to 

 establish uniformity in the typing of pneumococci and in the 

 standardisation of pneumococcal antisera. 



The immediate difficulty is that it does not seem possible to 

 reconcile this theory with the work of other observers, the special 

 feature of which is the absence of any demonstrable " mosaic " 

 in the pneumococcal antigen ; in their experience, agglutination, 

 precipitation and protection tests fail to show substantial affinity 

 between antibodies produced by one type and any hypothetical 

 antigenic components of another type. It is theoretically possible, 

 as I suggested on p. 49. that all pneumococci possess a common 

 antigenic nucleus, N, which is always linked with special antigens 

 a, b, or c, &c, in such a way that the special antigen controls 

 the reaction, e.g., N6 will only react with N6 antibody, not with 

 Na or Nc. But even this proposal, which postulates an un- 

 demonstrable N, would not serve to explain the differences 

 between the French school and other observers. 



For my own part, I think that the mosaic conception cannot 

 be applied in a literal sense or in amplified detail to pneumococci. 

 If, however, it implies no more than an array of biological 

 factors, many of which are unknown, it must be readily con- 

 ceded that research which may lead to fresh discoveries of such 

 factors is eminently desirable. 



On p. 55 it is suggested that some substances may lose 

 their " foreign " characteristic (i.e., their capacity for acting as 

 antigens) more readily than others when introduced into the 

 animal body. If the value of a good therapeutic serum for 

 parasitic bacteria depends on the presence of a special antibody, 

 distinct from agglutinins and precipitins, it is theoretically 

 possible that the difficulty of producing such sera in the case of 

 pneumococci may be associated with a high degree of instability 

 in the antigen required to produce this antibody. 



Coming to the question of physical influences and, in parti- 

 cular, to the significance of the fact that the interacting substances 

 are colloids, fresh difficulties arise, which, it would appear, tend 

 to confuse rather than to clarify the problem. Unfortunately, 

 these difficulties cannot be evaded. Owing to the importance 

 of colloidal conditions, it has become necessary to abandon the 

 conception of immunity as a purely chemical affair of receptors 

 and side-chains. These colloidal conditions cannot be treated 

 merely as conveniences which serve as a good excuse for getting 

 rid of some of the encumbrances of the side-chain theory ; they 

 are awkward and puzzling facts of an extremely obscure nature. 

 They imply that quantitative conditions play an important part 

 in immunity reactions, and that these reactions, owing to the 

 complexity of varying colloidal influences, do not follow ordinary 

 principles of chemical combination and dissociation. Hence the 

 idea of a " pure " pneumococcal antigen, which presents diffi- 

 culties from the standpoint of chemical structure alone, sc< 



