46 VACCINE AND SERUM THERAPY. 



tained for pooled normal serum he called 1.0, and the index de- 

 termined in this way he called the percentage index as contrasted 

 to Wright's bacillary index. These two indices Simon finds can 

 be made to agree to the second decimal place. 



Moss and others have found that the absolute numbers of 

 cells phagocytosing depends in normal or slightly diluted sera on 

 the strength of the bacterial suspension, and that in low dilutions 

 practically all the cells can be made to engulf bacteria. It is 

 evident that by Simon's method only sera having low indices can 

 be investigated, unless the bacterial emulsion or serum are di- 

 luted. Simon has recommended that light bacterial suspensions 

 and at times diluted sera be used so as to be able to determine 

 the percentage index. 



Strong, in his early experiments with anti-plague serum, 

 determined the index according to Wright's method. Later, 

 however, he substituted this method by one in which the highest 

 dilution of an immune serum which gives a marked phagocytosis 

 is compared to the same dilution of a normal serum. If, with a 

 certain dilution of an immune serum marked phagocytosis is ob- 

 served while the same dilution of a normal serum gives only slight 

 phagocytosis, the immune serum is regarded as having an increased 

 opsonin content. Even on this method Strong places little re- 

 liance. 



Although Simon prefers to accept his index to Wright's 

 when there is disagreement between the two, most investigators 

 have preferred Wright's opsonic index. 



LEUCOCYTES IN WHICH THE BACTERIA 

 ARE TO BE FOUND. 



According to Wright's method the bacteria are to be 

 counted in the polynuclear neutrophiles. These cells when 

 Wright's technique is followed will be found along the margins 

 and end of the spread. 



In all work on the determination of opsonic indices it has been 

 found that there are great differences in the number of bacteria 

 taken up by the individual leucocytes. Moreover, great differ- 

 ences have been found in the phagocytic index as determined by 

 different investigators examining the same slide. Even the same 



