74 



Proceeding from the chemical to the physical conditions which 

 determine immunity reactions, it must be recognised that colloidal 

 interactions in the living body are so complex that they are not 

 amenable to experimental analysis. Sometimes this difficulty 

 may be disregarded and it may suffice to assume, in general terms, 

 that immunity reactions depend upon the combined influences 

 of chemical affinity and physical adsorption. Thus, one may 

 speak of the neutralisation of toxin by antitoxin, or the 

 sensitisation of bacteria by agglutinins, precipitins or bacterio- 

 tropins, as simple facts due to the interaction of a definite antigen 

 with a definite antibody, each of which is assumed to exist as a 

 " pure " concrete substance. But, for the more difficult problems 

 concerning susceptibility and resistance to disease, no simple 

 explanation, such as the presence or absence of an appropriate 

 antibody, is forthcoming. In such cases it is necessary to widen 

 the field of enquiry, and here physical ideas about immunity 

 come in for consideration, including the idea of a " balanced 

 mechanism " which I have discussed at some length. Specu- 

 lations of this sort are not conclusive ; but they may be useful 

 as correcting the tendency to frame all working hypotheses in 

 terms of antigens and antibodies. 



This tendency is often noticeable in discussions about im- 

 munity. Though it is customary to introduce prefatory remarks 

 to the effect that very little is known about what takes place 

 in the living body, and that further knowledge of natural im- 

 munity would throw fresh light on acquired immunity, the really 

 business part of the discussion is frequently based on the maxim — 

 find the right antibody to the right antigen, and then complement 

 and phagocytosis will do the rest. But there are many facts 

 about immunity which, though admittedly difficult to under- 

 stand, at least indicate that this assumption needs revision. 



Pneumococcal Antigens and Antibodies. 



In applying the above general considerations to pneumococci, 

 one first wonders if they help to explain why this species, though 

 fairly well defined, contains such a large number of strains which 

 are antigenically distinct from each other. Perhaps they are 

 examples of condition (2) mentioned above (p. 49). If, in 

 addition to serological differences, there was also strong evidence 

 of serological relationship, the differences, or some of them, 

 might be attributed to the " masking " of antigenic constituents 

 exemplified in condition (4) ; but, since such serological inter- 

 relationship is not at all well marked, (2) seems preferable to (4) 

 in application to the pneumococcal species. One is reluctant 

 to think that each serologically different strain is really what 

 might be termed a distinct serological species, such as a species in 

 condition (1). 



Can the pneumococcus be changed in the living body from 

 one type into another ? It is theoretically possible that in 

 acute disease there may be a tendency towards mutation into 

 certain special types, and that in recovery there may be mutation 

 into " atypical " varieties ; but there is no satisfactory proof 



