67 



to become more and more of an abstraction when one remembers 

 that the real substance possessing antigenic characters is a 

 ^colloidal complex, that it is intimately dependent on its relations 

 to other colloids, and that its fate depends not on a single 

 reaction but on a sequence of events, wherein the resultant of 

 one reaction determines the reaction which follows. 



There is at least one definite conclusion to be derived from 

 the thought of all these complexities; the antigenic properties 

 of pneumococci which have been demonstrated by Neufeld and 

 his successors cannot lead to final conclusions, because they are 

 obviously very far from being the sum and substance of the 

 interactions which take place between pneumococci and the 

 living body. 



On pp. 57-59 I discussed the idea that chemico-physical 

 equilibrium is a factor which may considerably modify current 

 notions about antibodies. This attempt to correlate the pro- 

 perties of antibodies with what may be termed biological equi- 

 librium seems to imply that, though the importance of chemical 

 influences cannot be ignored, the individuality of an antibody is 

 to be regarded as the expression of a particular balance of colloidal 

 forces rather than the attribute of one particular chemical 

 structure. 



Is this line of thought worth considering in relation to 

 pneumococcal immunity ? I concede at once that it does not 

 directly point the way to new avenues for research ; but it may 

 serve as a useful corrective against ideas which tend to the 

 opposite extreme of explaining results as due to the interplay of 

 isolated factors, irrespective of their environment. The con- 

 ception of pneumococcal antibodies as hard and fast substances, 

 each with an individuality of its own, cannot be regarded as 

 established irrefutably and may be incompatible with colloidal 

 conditions of metabolism. 



These chemico-physical considerations suggest that the more 

 popular conceptions about immunity are inadequate, but do 

 not provide as an alternative any very helpful working hypo- 

 thesis, because colloidal reactions in the living body are too 

 complex for analysis. Still, it may be useful to be confronted 

 with these difficulties, if only to prevent one's ideas from 

 running in too narrow a groove. 



Therapeutic Value of Pneumococcal Antibodies. 



On pp. 59-63 I discussed possible limitations of the antigen- 

 antibody conception. It remains to consider whether these 

 suggestions are applicable to pneumococci. 



To begin with what is known about the properties of pneumo- 

 coccal antibodies the existence of which has been demonstrated 

 experimentally, the results of protection tests on mice have made 

 it obvious that certain specific substances prevent the corre- 

 sponding variety of pneumococcus from gaining an initial foot- 

 hold in the animal body. Here two assumptions might be made. 



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