INFECTION AND RESISTANCE 



nique uncontrollable sources of error influence this method. Most 

 important among them are the differences necessarily existing be- 

 tween different normal sera used for comparison and differences 

 in the agglutinative powers of the sera used in the two specimens. 

 For it is plain that different degrees of agglutination may bring 

 about great variations in the number of bacteria with which the 

 individual leukocyte comes into contact. 



Wright's method has also been particularly unsatisfactory in 

 taking the opsonic index against such bacteria as the typhoid bacillus 



and the cholera spirillum, or- 

 ganisms which are very rap- 

 idly digested after being 

 taken up by the leukocytes. 

 In consequence, even after as 

 short an incubation time as- 

 five or ten minutes, the in- 

 gested bacteria are partly dis- 

 integrated, are stained in- 

 distinctly, and cannot be 

 counted with accuracy. In 

 order to avoid this source of 

 error Klien 6 has devised a 

 modification which depends 

 upon gradual dilution of the 

 serum in a series of pha- 

 gocytic tests with the 

 LEUKOCYTES CONTAINING BACTERIA. DRAW- same leukocytic and bacterial 

 ING OF FIELD AS SEEN IN WEIGHT'S , emulsions. In this way he 

 METHOD OF OPSONIC-INDEX ESTIMATION. , . , , %. ,. 



determines the degree 01 di- 

 lution of the serum to be 



tested at which phagocytosis no longer exceeds that taking place in 

 salt solution alone. The degree of dilution at which this result was- 

 obtained has been called by Simon the "coefficient of extinction." A 

 comparison of sera with regard to this value, it is clear, furnished an 

 estimate of their quantitative opsonic properties quite as instructive 

 as the direct estimations by the Wright method, and in our opinion, 

 at least, more reliable. Though also subject to some of the objections 

 advanced against the Wright method, it has the definite advantages 

 mentioned above, and is not so closely dependent upon irregularities 

 in counting, agglutinin influences, and differences in relative pro- 

 portions of bacteria and leukocytes employed. Jobling 7 has used 

 this method with success for the standardization of antimeningitis 

 serum. 



A further modification suggested by Simon, Lamar, and Bis- 



6 Klien. Johns Hop. Hosp. Bull, Vol. 18, 1907. 



7 Jobling. "Studies from the Rockefeller lust.," Vol. 10, 1910, p. 614. 



