64 



structural differences, such as are termed, on p. 48, differences 

 in the " dominant antigen." I am not suggesting that this 

 latter hypothesis can be excluded, but merely note that it does 

 not appear to be necessary. 



\\ ith some bacterial species, different races are found to 

 possess certain serological attributes in common, though differing 

 in other serological respects. This overlapping of the serological 

 reactions is naturally explained, as illustrated on p. 49, by the 

 postulate of multiple antigenic components. It is also probable 

 that, in strains belonging to such species as these, common 

 antigenic constituents may sometimes be present, though failing 

 to respond to the appropriate antibody, as in the example 

 (p. 50) of concealed antigenic constituents. But when, as is 

 the case with different races of pneumococci, serological inter- 

 relationship is not demonstrable to any conspicuous extent, 

 there is less support for the idea that such " masked " antigenic 

 constituents are a common property of the variants. Still, this 

 view is theoretically possible. Hence the justification for the 

 working hypothesis to which some investigators still adhere, 

 that serological reactions will ultimately be found which will 

 unmask these concealed constituents and demonstrate sero- 

 logical properties common to all pneumococci. This possibility 

 may be taken as a reason for saying that the present method of 

 serological classification need not be taken as final, though the 

 way of finding a better method may not be clear. 



Dealing with the position as it now stands, there appears to 

 be an indefinitely large number of variants, and therefore it 

 may be impossible to make an exhaustive serological classifica- 

 tion by means of agglutinins. The attempt may be made to 

 classify as many strains as possible into " races " and thereby 

 to reduce the unclassifiable strains to a minimum ; but one 

 doubts if it would be profitable to follow up this line of enquiry 

 very far. 



Hence many investigators, though not all (see pp. 14-18 of 

 my preceding report), have abandoned the attempt to make a 

 polyvalent or omnivalent serum which would contain the special 

 antibody for each variant. They think it more useful to con- 

 centrate attention on the relatively small number of varieties 

 which comprise the majority of the strains isolated from cases 

 of lobar pneumonia. 



Chemical and Colloidal Instability. 



The questions of " chemical instability " discussed above 

 (pp. 50-51) lead one to consider whether the different antigenic 

 variants of pneumococci are really " fixed " and sharply or 

 permanently distinguishable, one from another. 



Some laboratory evidence of antigenic relationship has been 

 demonstrated between different serological variants; but, as 

 I have already remarked, such resemblances appear to be of 

 minor character, the more striking feature being that the variants 





