70 



from other antibodies much in the same way as one antigen 

 differs from another. This view, as I have suggested above 

 (pp. 57-59) in discussing " colloidal balance," may turn out to 

 be inadequate. And there are other reasons for not attaching 

 exclusive importance to the protection afforded by specific 

 antibodies. 



In the case of an animal which is naturally immune against 

 all varieties of pneumococci, how are the different antigenic 

 variants disposed of? For reasons discussed above in relation 

 to the " mosaic " theory (pp. 51-54), it does not seem likely 

 that such an animal is provided with an indefinitely large supply 

 of antibodies, each of which corresponds exactly with one or 

 other of the antibodies produced artificially by immunisation 

 with a variant. It seems more probable that the antigenic 

 complex as a whole is attacked in some other way, and that 

 some vulnerable point is found which has nothing to do with the 

 particular structure characteristic of the variant. 



In other species of animals, where natural resistance does 

 not amount to absolute immunity, is spontaneous recovery 

 from such a disease as lobar pneumonia due to the elaboration 

 of specific antibodies against the variant causing the infection, 

 or is it due to an independent mechanism, similar to that of the 

 naturally immune animal though less rapidly effective ? Probably 

 the latter factor is of major importance, as it would be difficult 

 to maintain that, at the time of the crisis, the development of 

 specific antibodies against the particular variant is sufficiently 

 pronounced to be regarded as the primary cause of recovery. 



It must be admitted that the above arguments are not con- 

 clusive, because the future may lead to the discovery of some 

 new kinds of antibodies which will provide the explanation 

 required. But, in the meantime, it is desirable to call attention 

 to some other aspects of the problem. 



What is the explanation of the more important facts in the 

 life history of pneumococci, such as the change from a sapro- 

 phytic to a parasitic mode of life and the production of various 

 forms of disease ? Though at present the explanation is unknown, 

 it at least seems clear that one cannot hope to arrive at it by 

 simply ringing the changes on one idea, the antigenic stimulus 

 of a foreign protein. 



It is equally unlikely that the explanation of immunity 

 towards pneumococci will ever be summed up as due to 

 " pneumococcal antibodies," unless the term " antibody " receives 

 some entirely new interpretation. Antibodies must be helped 

 out by appealing to the aid of the mysterious forces known as 

 " vital resistance." On pp. 62-63 I have discussed briefly 

 attempts to simplify this mystery by identifying two of the 

 forces with complement and phagocytosis. It is not likely that 

 any strong claims will be made for " complement " as explanatory 

 of resistance against pneumococci. But phagocytosis is often 

 regarded as of high importance in the defences of the body against 



