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tion why immunisation of horses with the second, and third 

 of the " fixed " types has failed to produce a good therapeutic 

 serum, when immunisation with the first has succeeded. 



The Americans appear to have demonstrated sharp distinc- 

 tions between their Types I, II and III, and to this extent there 

 is a justification for their classification; but for the remaining 

 strains it seems to have been found that typing is either impossible 

 or can only be managed by making an indefinitely large number 

 of groups. It would be unsafe to assume that these " atypical " 

 strains are of relatively small importance ; they are too numerous 

 and too frequently responsible for cases of pneumonia. 



The diversity of antigenic structure amongst strains of pneumo- 

 cocci presents a difficulty which has not yet been overcome. 

 Though there is some occasional evidence of immunological 

 relationship between strains which present serological differences, 

 this is no proof that such differences are negligible ; some of them 

 certainly appear to be important and possibly all of them are ; 

 at all events it has not been found possible to discriminate between 

 those which are important and those which are not. 



Both Preston Kyes and the French school hope that they 

 have found ways out of these difficulties, but their claims are 

 in need of further confirmation. 



Conclusions. 



1. Results obtained from a continuation of the Rockefeller 

 fine of investigation will be awaited with interest; at present, 

 there appear to be obstacles which render further progress 

 difficult. 



2. At the same time, other methods of attempting to make 

 therapeutically useful antipneumococcal sera are worth con- 

 sidering, as it cannot be taken as proved that the Rockefeller 

 method is the only one by which it is possible to attain success. 



3. With certain parasitic bacteria, of which anthrax and fowl 

 cholera appear to be examples, therapeutically potent antisera 

 have been obtained, although the mechanism of their action 

 is not clearly understood. It is just possible that a comparative 

 study of the serological reactions of one of these bacteria and the 

 pneumococcus might give some clue to an improved method of 

 preparing antipneumococcal sera. 



4. The study of recent work on pneumococci raises many 

 difficult questions concerning the underlying principles of 

 immunity reactions. A reconsideration of some of these general 

 principles is necessary as an adjunct to experimental enquiry. 



