AGGLUTININS 263 



strength. Hence the serum must always be diluted with 

 physiological salt solution (0.85 per cent). Further, closely 

 related bacteria may be agglutinated to some extent by the 

 same serum. It is evident that if they are closely related, 

 their protoplasm must contain some substances of the same 

 kind to account for this relationship. Since some of these 

 substances may be agglutinogens, their introduction into 

 the animal body will give rise to agglutinins for the related 

 cells, as well as for the cell introduced. The agglutinins 

 for the cell introduced "chief agglutinins" will be formed 

 in larger quantity, since a given bacterial cell must contain 

 more of its own agglutinogen than that of any other cell. By 

 diluting the blood serum from the animal to be tested the 

 agglutinins for the related organisms (so-called "coagglu- 

 tinins" or "partial agglutinins") will become so much 

 diminished as to show no action, while the agglutinin for 

 the specific organism is still present in an amount sufficient 

 to cause its clumping. Agglutinins are specific for their 

 particular agglutinogens, but since a given blood serum may 

 contain many agglutinins, the serum's specificity for a given 

 bacterium can be determined only by diluting it until this 

 bacterium alone is agglutinated. Hence the necessity of 

 diluting the unknown serum in varying amounts when test- 

 ing against several known bacteria to determine for which 

 it is specific, i. e., which is the cause of the disease in the 

 animal. 



The agglutinins in the serum may be removed from it 

 by treating it with a suspension of the cells for which agglu- 

 tinins are present. If the "chief" cell is used all the agglu- 

 tinins will be absorbed. If related cells are used, only the 

 agglutinins for this particular kind are removed. These 

 "absorption tests" furnish another means of determining 

 specificity of serum, or rather of determining the "chief 

 agglutinin" present. 



Just as an unidentified disease in an animal may be deter- 

 mined by testing its serum as above described against hioion 

 kinds of bacteria, so unknown bacteria isolated from an 

 animal, from water, etc., may be identified by testing them 

 against the blood sera of different animals, each of which has 



