OPSONINS TROPINS BACTERIAL VACCINES 167 



essentially in mixing intimately equal volumes of bacterial emulsion, 

 serum, and leukocytes; after incubation at body temperature the 

 mixture was spread evenly upon microscopic slides, stained, and 

 examined with the microscope. The average number of bacteria per 

 polymorphonuclear leukocyte was determined by direct count. A 

 comparison, under parallel conditions, of the phagocytic activity of 

 leukocytes for a specific organism in the serum of a normal individual 

 and that of an individual infected with the specific organism, accord- 

 ing to the technic outlined below, was called by Wright the opsonic 

 index. 



Procedure. 1. Leukocyte Suspension. About 0.5 c.c. of blood, 

 drawn from the lobe of the ear or the tip of the finger, is collected in 

 a centrifuge tube containing 10 c.c. of sterile physiological salt solu- 



FIG. 9. Phagocytosis of streptococci. 



tion in which has been dissolved 1 per cent, of sodium citrate; this 

 mixture is centrifuged at moderate speed until a sharp separation 

 of blood cells and clear supernatant fluid is obtained. The super- 

 natant fluid is carefully poured off and the top layer of blood cells, 

 which contains practically all the leukocytes, is removed to a fresh 

 centrifuge tube containing 10 c.c. of physiological salt solution. 



A second centrifugalization is made, and again the supernatant 

 fluid, containing the last traces of blood serum, is discarded. The 

 sediment, rich in leukocytes, is used as the leukocyte suspension in 

 the test. 



2. Suspension of Bacteria. Bacteria from a culture on solid media 

 are suspended in sterile salt solution and agitated until a fine opales- 

 cent emulsion is obtained. This is most conveniently accomplished 



