264 RECEPTORS OF THE SECOND ORDER 



been properly inoculated with a different kind of known 

 hacteriwn. 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 has been inoculated, or immunized, as is 

 usually stated. This method of identifying cultures of bac- 

 teria 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 sufficient, as in the "intestinal group" of organisms in 

 human practice. 



The diagnosis of disease in an animal by testing its serum 

 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 

 diagnosis (Malta fever), in most cases, however, 50 times 

 is the lowest limit. Evidently the greater the dilution, that 

 is, the higher the "titer," the more specific is the reaction. 



The agglutination test is also one way of determining 

 whether two human bloods belong to the proper class when 

 it is desired to transfuse from one person to another. There 

 are four classes of blood with respect to the agglutination 

 between the red corpuscles and the plasma or serum. The 

 test is likewise of some value in determining the parents of a 

 child. 



PRECIPITINS. 



Since agglutinins act on bacteria, probably through the 

 presence of substances within the bacterial cell, it is reason- 

 able to expect that if these substances be dissolved out of the 

 cell, there would be some reaction between their (colloidal) 

 solution and the same serum. As a matter of fact, Kraus 

 (1897) showed that broth cultures freed from bacteria by 

 porcelain filters .do show a precipitate when mixed with the 

 serum of an animal immunized against the particular bac- 

 terium and that the reaction is specific under proper condi- 

 tions of dilution. It was not long after Kraus's work until 



