244 RECEPTORS OF THE SECOND ORDER 



cells, as well as for the cell introduced. The agglu- 

 tinins for the cell introduced will be formed in larger quan- 

 tity, 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 " coagglutinins") 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 testing against several known bacteria to 

 determine for which it is specific, i. e., which is the cause 

 of the disease in the animal. 



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

 mined by testing its serum as above described against known 

 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 

 been properly inoculated with a different kind of krwwn bac- 

 teria. If the unknown organism is agglutinated by the blood 

 of one of the animals in high dilution, and not by the others, 

 evidently the bacterium is the same as that with which the 

 animal had been inoculated, or immunized, as is usually 

 stated. This method of identifying cultures of bacteria is 

 of wide application, but is used practically only in those 

 cases where other methods of identification are not readily 

 applied, and especially where other methods are not suffi,ci-ent, 

 as in the "intestinal group" of organisms in human 

 practice. 



The diagnosis of disease in an animal by testing its sermn 

 is also a valuable and much used procedure. This is the 

 method of the "Widal" or "Gruber-Widal" test for typhoid 

 fever in man and is used in veterinary practice in testing 

 for glanders, contagious abortion, etc. In some cases a dilu- 

 tion of the serum of from 20 to 50 times is sufficient for 



