PREFACE TO THE FOURTH EDITION 



THE favorable reception accorded to the previous 

 editions of this book, together with the fact that 

 our knowledge of the subject has increased con- 

 siderably in the past few years, has led the author 

 to undertake a thorough revision of the work. 



In its first edition, in 1904, this book dealt only 

 with certain antibodies whose discovery had aroused 

 a great deal of scientific interest, namely, hsemoly- 

 sins, cytotoxins, and precipitins. To this was 

 added, in subsequent editions, a discussion of anti- 

 toxins, agglutinins, and opsonins. All these topics 

 were naturally embraced under the title " Immune 

 Sera." In the present (fourth) edition, while the 

 old title has been retained, the scope of the sub- 

 ject matter has been greatly extended, so that now 

 there is presented an exposition of the main facts 

 of infection and immunity. 



It is but natural that any discussion of the 

 immunity reactions should center about the ingen- 

 ious side-chain theory of Ehrlich, which has domi- 

 nated the work in this field. Its heuristic value 



