DISEASE, INFECTION, AND EESISTANCE 299 



on top. The white blood corpuscles are intermediate and 

 will appear in large numbers in the surface layers of the 

 sediment. The serum may be then pipetted away, the white 

 blood cell layer removed by means of a small spoon or 

 spatula, resuspended in physiological salt solution and 

 centrifuged once or twice again to wash away all of the 

 serum. The cells are then suspended in physiological salt 

 solution for study. 



A suspension of the bacteria to be studied is also prepared 

 in physiological salt solution. A suitable dilution of the 

 serum to be tested is also prepared. By means of a capillary 

 pipette a small amount of the suspension of white blood 

 cells, and equal amounts of the bacteria suspension and of 

 the serum to be tested are drawn up, then blown out upon 

 a glass slide and mixed. They are then drawn again into 

 the pipette, the end sealed and placed in an incubator at 

 371/^° C. for fifteen minutes. The mixture is then blown 

 again onto a glass slide, spread, fixed and stained. In 

 general there will be a direct relationship between the 

 amount of opsonin specific for the organism used present in 

 the serum and the number of bacteria engulfed on the 

 average by white blood corpuscles. If bacteria and white 

 blood cells alone are mixed without serum containing 

 opsonin, very few if any of the bacteria will be engulfed. 



Opsonic Index. — It is possible to compare the opsonin 

 content of one individual and that of another by determina- 

 tion of what is termed the opsonic index. This is a number 

 which represents roughly the ratio between the amount of 

 opsonin contained in the two bloods. "When white blood 

 corpuscles, bacteria and serum from a patient, for example, 

 are tested and the average number of bacteria engulfed per 

 white blood cell determined, and the same process carried 

 out with the same bacteria, white blood cells and serum from 

 a normal individual, the ratio of the average number of 

 bacteria engulfed as the result of the use of patient serum 



