INFECTION AND IMMUNITY. 579 



the purpose of ascertaining the identity of bacteria, and 

 in applying this diagnostic test for this purpose it is 

 necessary to know the limits of the agglutinating power 

 of the serum for the organism employed in immunizing 

 the animal, so as not to be misled by the presence of 

 relatively large amounts of the common agglutinins. 1 



The agglutinating properties of the blood serum of 

 an immune animal are not always in proportion to its 

 protective or curative properties. There may be a rela- 

 tively high degree of agglutination without a corre- 

 sponding bactericidal action of the serum. From this 

 fact the agglutinating properties of a serum cannot be 

 taken as a basis of its value when employed therapeu- 

 tically. The exact relation between the agglutinating 

 and the bactericidal properties of the blood serum of an 

 animal cannot be stated, though it seems probable that 

 they are both the result of certain reactive processes on 

 the part of the tissue-elements against the bacteria. 



THE MECHANISM OF IMMUNITY. The problem 

 involving the explanation of the interesting ideas and 

 observations with regard to immunity has afforded 

 material for reflection and hypothesis for a long time. 

 It is only through investigations conducted during recent 

 years that it has met with anything approaching satis- 

 factory solution, and even now there remain a number 

 of important points that are veiled in obscurity. 



Conspicuous among the observers who have endeav- 

 ored to explain the mechanism of immunity may be 

 mentioned Chauveau, Pasteur, Metchnikoff, Buchner, 

 Fliigge and his pupils (Smirnow, Sirotinin, Bitter, 



1 A great deal of work has been done in recent years on the common 

 and specific agglutinins in dysentery immune serum by Park and his 

 associates, as well as by others (see Journal of Medical Research, vols. 

 v., vi., and vii.). 



