Chapter XIII 



OPSONIC INDEX 



The frequent mention of the term "opsonic 

 index" in the literature makes it imperative that 

 the physician have a general idea of the technic 

 and significance of the reaction. The opsonic 

 content of a given serum is estimated by com- 

 parison with normal serum. By the opsonic in- 

 dex is meant the ratio of the average number 

 of bacteria per leukocyte taken up in the pres- 

 ence of a given serum, to the average number 

 of bacteria taken up in the presence of normal 

 serum. 



Historic Metchnikoff discovered that certain 

 leukocytes play a most important part in the 

 process of immunity as phagocytes, by their 

 property of taking into themselves and destroy- 

 ing pathogenic bacteria. Wright and Douglas 

 showed definitely that phagocytosis of bacteria 

 in serum depends upon the presence in the serum 

 of a special substance which acts on the bacteria 

 and sensitizes or prepares them for the leuko- 

 cytes. They called this substance opsonin. 



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