CHAPTER XIX 



THE PHAGOCYTIC POWER OF THE BLOOD AND THE 

 OPSONIC INDEX 



From the time that Metchnikoff connected the phenomena 

 of phagocytosis with those of immunity, there was no recognized 

 technic for the observation and comparison of the bacteria-con- 

 suming and bacteria-destroying power of the cells until 1902, when 

 Leishman* suggested the following simple method: 



A thin suspension of bacteria in normal salt solution is mixed 

 with an equal volume of blood by drawing in and out of a capillary 

 tube, then dropped upon a clean slide, covered carefully, placed in a 

 moist chamber, and incubated at 37°C. for a half hour. The cover 

 is then slipped off carefully, as in making blood-spreads, dried, 

 stained, and the number of bacteria in each of 20 leukocytes counted 

 and averaged. For comparison with the normal, the patient's 

 blood and normal blood are simultaneously examined. 



This was greatly improved by Wright and Douglas, f the accuracy 

 of whose methods enabled them to discover the "opsonins," work 

 out the "opsonic index," and formulate methods by which suffi- 

 ciently accurate observations could be made for controlling the spe- 

 cific treatment of infectious diseases. 



The opsonic theory teaches that the leukocytes are disinclined 

 to take up bacteria unless they are prepared for consumption or 

 phagocytosis by contact with certain substances in the serum that 

 in some manner modify them. This modifying substance is the 

 opsonin (opsono, I cater to, I prepare for). 



To make a test of the opsonic value of the blood it is necessary 

 to prepare the following: 



A uniform suspension of bacteria. 



A suspension of washed leukocytes in physiological salt solution. 



The serum to be tested. 



A normal serum for comparison. 



The Bacterial Suspension. — This is prepared like the similar 

 suspensions used for determining agglutination, but with greater 

 care, since the bacteria taken up by the corpuscles are to be counted, 

 and any variation in the number of bacteria with which they come 

 into contact may modify the count. It is also necessary to avoid 

 all clumps of bacteria for the same reason. 



The culture is best grown upon agar-agar for twelve to twenty- 

 four hours, the bacteria in young cultures being more easy to sepa- 



* "Brit. Med. Jour.," Jan. 11, 1902, i, p. 73. 

 f'Proc. Royal Soc. of London," 1904, lxxxii, p. 357. 

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