PREFACE TO THE FIRST EDITION XV 



number of particular problems connected with 

 immunity, which still await solution ; nor is there any 

 doubt that many well-ascertained facts are closely 

 connected with the results of our researches. Even 

 now it would be tempting to select suitable instances 

 from the mass of my particular observations, with a 

 view to giving a more general signification to the 

 views I have formulated on the means of defence 

 possessed by the organism against the invasion of 

 substances or cells that are out of harmony with the 

 body. For the time being I have refrained from 

 doing so, as the mere enumeration of closely related 

 observations, quite apart from a discussion of all the 

 hypotheses put forward, would enormously increase 

 the scope of this little volume, and incidentally would 

 interfere with a clear insight into our subject. 



Again, it is very difficult, for those not actually 

 engaged in research work on immunity, to keep in 

 touch with all the communications made at different 

 times concerning ideas and theories that are con- 

 stantly changing, and above all to find a sure footing 

 amongst the somewhat pleonastic terminology and 

 nomenclature employed. Theory and actual fact 

 form, in this field of research, a closely interwoven 

 net of conceptions ; so much so, that only those, who 

 have already acquired, by actual corroboration, a 

 thorough knowledge of all the problems connected 

 with the subject, are able to trace sharp limits 



