556 BACTERIOLOGY. 



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. 



IMMUNITY : HISTORIC SKETCH. In the course of 

 our studies aimed to secure light on the mechanism of 

 infection, two phenomena are constantly in evidence, no- 

 tably first, that not all animals are susceptible to infec- 

 tion by all pathogenic bacteria, and next, that an animal 

 which has recovered from inoculation with a pathogenic 

 species, to which it is by nature susceptible, has under- 

 gone a change that, as a rule, renders it insusceptible to 

 subsequent inoculations with the same species. Animals 

 in either the one or the other state are said to be 

 immune ; in the former to be immune by nature, in the 

 latter to have acquired immunity. 



In its present development there is no more fascina- 



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.). 



