PHAGOCYTOSIS 263 



Another modification of Wright's method, introduced by 

 Simon, concerns the method of counting only. A large number 

 of leucocytes are counted, and are classified simply into those 

 that contain bacteria and those that are free. Of course, the 

 emulsion must not be too thick, or practically all the leucocytes 

 will have taken some up. The process is repeated with the 

 control, and the results compared ; thus, if in the control film 

 25 per cent, of leucocytes were empty, and in the patient's film 

 50 per cent, the index would be ff = o > 5. A comparison of the 

 results obtained by this method and by careful counting show 

 that they are fairly comparable, and the process may be used 

 where it is only necessary to determine whether the index is 

 high or low. 



Another and more important method is that of dilution or 

 extinction, as introduced by Dean and by Klien. It is especially 

 useful in the case of bacteria, such as B. typhosus and V. cholera, 

 which are dissolved by fresh serum when but slightly diluted. 

 Further, when an attempt is made to determine the opsonic index 

 to the former, and the pipette is incubated for but five minutes, 

 numerous shadows and partially digested bacilli are seen within 

 the leucocytes, thus introducing a new and very important error. 

 In order to avoid this, Klien determines the degree of dilution of 

 the serum necessary for the complete extinction of its opsonic 

 action. In preparations in which no serum is used the phagocytic 

 index is usually below 0*5, and the serum to be tested is diluted 

 until the degree of dilution is found, which gives a phagocytic 

 index no higher than this. Working by this method, Klien 

 obtained results very different from those obtained by Wright's 

 method. In the process of immunization of a rabbit the index 

 (by the latter method) remained low, varying only between 0-82 

 and 1-65, whereas by the process of dilution it was seen to be 

 actually greatly raised. Before the commencement of the im- 

 munization the opsonic power of the serum was extinguished 

 when the latter was diluted thirty times, whereas afterwards it 

 did not disappear until diluted 3,072 times. It appears clear that 

 in the case of bacteria like this the results obtained by Wright's 

 method are quite misleading. Klien states that the bacterial 

 emulsion should be a thick one, and should be of about the same 

 density in successive experiments, if these are to be comparable. 

 The main objection to this method is its tediousness: many 

 pipettes have to be prepared, and many films examined. 



