MICHIGAN ACADEMY OF SCIENCE. 
113 
DIRECTIONS FOR DETERMINING THE OPSONIC INDEX OF THE 
BLOOD. 
By E. C. L. Miller, M. D. 
[From the Research Laboratories of Parke, Davis & Co.. Detroit, Mich.] 
The subject of immunity is being studied as never before. It is recognized 
to-day as the subject of paramount interest in medicine. Thanks to the genius 
of certain English workers, we are now able to make a quantitative deter- 
mination of at least some of the factors which go to make up immunity. 
Heretofore, the only test for immunity consisted in inoculating the animal 
with the germ in question, and from the amount of culture which it could 
withstand we tried to form some idea of the degree of immunity. Now we 
are able to measure, in vitro, at least approximately, one factor in immunity 
— phagocytosis. The method is simple, and, fortunately, applicable to man as 
well as to the lower animals. This fact at once makes it of interest to the 
physician, for by this means he is brought one step closer to his patient’s ac- 
tual vital condition. Methods have also been discovered by which the degree 
of immunity toward any particular microorganism can be increased, and 
thus is placed in the practitioner’s hands the means by which he can assist 
nature in resisting certain infections. 
The opsonic index of a patient indicates the efficiency of his phagocytic 
defense against any particular microorganism as compared with the average 
normal person. Preliminary to making a determination of the opsonic index 
it is necessary to prepare (1) an emulsion of the bacteria in question, (2) a 
quantity of washed blood corpuscles, (3) a drop or two of the blood serum 
of the patient, and (4) a little normal pool serum. With these constituents 
in hand the actual test is made as follows: 
Equal portions of the emulsion of bacteria, the washed blood corpuscles 
and the pool serum are mixed and incubated. During this incubation some 
of the bacteria are taken up by the phagocytes. Then smears of this mix- 
ture are stained and examined with the microscope in order to determine 
to what extent the bacteria have been taken up. The number of bacteria 
in the phagocytes is systematically noted as they come into view, and when 
50 or 100 leucocytes have been thus counted the average is determined. 
This average is the phagocytic count when using the patient’s serum. 
An exactly similar test, carried through by using blood corpuscles, bac- 
terial suspension and pool serum in equal quantities, gives the phagocytic 
count when using pool serum. The opsonic index is found by dividing the 
phagocytic count of the patient’s serum by the phagocytic count of the 
pool serum. 
An opsonic index much below normal usually indicates inadequate power 
of resistance on the part of the patient toward the bacterium in question, 
and this power can be increased by the administration of homologous bac- 
terial vaccines; a bacterial vaccine in this sense being simply a suspension 
of the bacteria in physiological salt solution, sterilized properly and stan- 
dardized. A bacterial vaccine may be made from a pure culture obtained 
from the patient himself (in which case it is spoken of as an autogenous 
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