OPSONIC INDEX AND VACCINE THERAPY 331 



normal individuals. By the use of such a serum mixture any slight 

 possible variations from the normal in any one of the sera are likely 

 to be equalized, and a closer approach to a normal standard is at- 

 tained. 



The leukocytes used in both tests are the same and taken, as a 

 rule, from the blood of the worker or from some other supposedly 

 healthy person. They are obtained by taking 15 or 20 drops of 

 blood from the finger or ear into 5 to 10 c. c. of sodium citrate solu- 

 tion, in which the blood does not clot. Brief centrifugalization throws 

 down the blood cells, with a thin, buffy coat of leukocytes on top, and 

 these are gently taken off with a pipette. This constitutes the leu- 

 kocytic cream of Wright's experiments, and furnishes a uniform leu- 



METHOD OF TAKING UP EQUAL VOLUMES OF LEUKOCYTES, BLOOD SERUM AND 

 BACTERIAL EMULSION IN WRIGHT'S TECHNIQUE FOR OPSONIC-INDEX DETER- 

 MINATION. 



kocyte factor for the two tests which are to be compared. The bac- 

 teria are obtained by emulsifying carefully in salt solution. It is 

 very important to obtain an emulsion free from clumps and neither 

 too thick nor too thin, a result which can be secured only by experi- 

 ence. 



Equal quantities of serum (unknown and normal "pool" respec- 

 tively) are mixed with equal quantities of the bacterial emulsion and 

 the leukocytes in capillary pipettes, and the mixtures are incubated 

 for fifteen to thirty minutes under exactly similar conditions. At 

 the end of this time smears are made upon slides, the preparations 

 stained, and the numbers of bacteria in a hundred or more leuko- 

 cytes counted in each of the two experiments. The average is taken, 

 and from the phagocytic indices thus obtained the opsonic index is 

 calculated. For instance, if 



Phagocytic index (normal pool) = 8 

 Phagocytic index (patient's serum) = 6 



then the opsonic index (patient's serum) = 0.75. Or, if the phago- 

 cytic index of the normal pool had been 10. and that of the patient's 

 serum 15., then the opsonic index of the patient's serum, higher than 

 normal, would be 1.5. 



For the insurance of accuracy in carrying out this method Wright 

 calls especial attention to the caliber of the capillary pipettes that 

 are used, the concentration of the sodium citrate solution, which 

 should be 1.5 per cent, and the freshness of the leukocytes. " But 

 it is still necessary to remember that with the greatest care in tech- 



