AGGLUTININS AND PRECIPITINS 



drop and so on until the series of dilutions has been made, discarding a loop- 

 ful of the last mixture and leaving one loopful of salt solution as a control, 

 then adding to each drop a loopful of bacterial suspension. The slips or 

 slides are inverted, sealed, incubated and read. In using slides, the trouble 

 of sealing may be avoided by incubating in a moist chamber. The micro- 

 scopic method is usually employed in the Widal test, the dilutions of patients' 

 blood or serum being made by the same drop method, 1-20, 1-40, 1-80. Some- 

 times a drop of dried blood is used, this being laked and dissolved by a drop 

 of water and then made up to the first dilution of 1-20 by the addition of 

 nineteen drops of saline. Frequently twenty-four-hour broth cultures of the 



FlG. 8. The Wright tube for obtaining small 

 quantities of blood serum. 



FIG. p. Coiled pipette for taking up small quantities of fluids. Bubbling in the 

 coil gives warning of the filling and prevents suction into the mouth. The tube may 

 be made straight and plugged with C9tton. Either may be used for withdrawing 

 serum from the Wright pipette. Suction may be applied by the mouth directly or 

 with a rubber tube or by means of a small nipple. 



typhoid bacillus are employed as the emulsion. Clearer results, however, are 

 obtained by collecting blood in Wright tubes (Fig. 8) and allowing the 

 serum to separate for dilution, and then employing a salt solution suspension 

 of a twenty-four-hour agar slant culture. 



Specificity of Agglutinins Group Reactions. >The specificity 

 of the reaction may be shown by setting up dilutions of the anti- 

 typhoid serum obtained from the immunized rabbit against suspen- 

 sions of bacillus typhosus, bacillus paratyphosus (A or B), and 

 bacillus coli communis. An illustrative protocol follows : 



This protocol illustrates two points, first, that the serum agglu- 

 tinates its homologous bacteria in high dilutions, and second, that 

 in strong concentrations it also agglutinates other organisms of the 

 same group. Thus the specificity is not absolute throughout, but 

 there is a " zone of absolute specificity," in this case between the 



