262 OPSONIC TECHNIQUE 



index as determined by Wright's method might nevertheless be 

 normal ; yet this blood might have but little power of destroying 

 bacteria which gain access thereto. There is some experimental 

 evidence that alterations in the power of the leucocytes do 

 actually occur ; thus Shattock and Dudgeon, in some experiments 

 with granules of melanin (which, like bacteria, require to be 

 opsonized before they can be taken up by leucocytes), found that 

 either more or less might be taken up by the patient's leucocytes 

 as compared with normal ones, using the same serum in all cases. 

 The numbers varied between 0-46 and 2*9, taking the normal 

 number as unity. It must be pointed out that this method does 

 not give the opsonic index of the serum, and that in cases, e.g., 

 in which a low result is obtained it affords no information as to 

 whether the leucocytes or the serum is at fault, or both. Further, 

 there is a possible error owing to the possible difference in the 

 number of the leucocytes in the unit volume of the two specimens 

 of blood. Where the patient has a leucocytosis and this is very 

 common in the type of case in which opsonic estimations are 

 required the difference may be very great. The result of this 

 has not been fully elucidated, but it is obvious that where the 

 bacterial emulsion is not very thick the number available per 

 leucocyte is very different in the two cases. This is a point 

 worthy of consideration in the determination of the opsonic index 

 by Wright's method. 1 When the bacterial emulsion is very 

 dilute a large error is introduced, and even if very large numbers 

 of leucocytes are counted the results are untrustworthy. The 

 best results theoretically would be obtained where the emulsion 

 was so thick that every leucocyte would take up as many bacteria 

 as it was capable of doing in the given time. This is impracticable, 

 however, as the labour in counting leucocytes containing very 

 many bacteria is great, and the error in counting is also large. 

 Probably the best results are obtained where the phagocytic 

 index in the control is about 4, and it is a good plan to 

 perform an orientating experiment to determine the appropriate 

 strength of the emulsion before commencing a large series of 

 opsonic determinations. 



1 It has been investigated by Ruth Tunnicliffe, who finds no very great 

 differences in a series of estimations in which the bacteria (diphtheria bacilli) 

 varied from 125,000 to 1,000,000 per cubic millimetre, all the other factors 

 being constant ; and by Walker, who finds that the index rises greatly if a 

 thicker emulsion is employed. 



