FACTORS DETERMINING PHAGOCYTOSIS 313 



when Leischman 8 introduced a technique by means of which it be- 

 came possible to observe the process of phagocytosis with fresh serum 

 and leukocytes in vitro. 



By utilizing this technique and improving upon it Wright and 

 Douglas in the following year (1903) evolved a method by means of 

 which phagocytic activity could be quantitatively measured with 

 reasonable accuracy. They worked at first with staphylococcus 

 phagocytosis by human leukocytes in the presence of human citrate 

 plasma, a research undertaken primarily because Wright, 9 in collabo- 

 ration with Windsor, had previously determined that human blood 

 serum possessed practically no bactericidal power for this organism, 

 and that phagocytosis was probably the chief mechanism of protection 

 which the human body possessed against these bacteria. The re- 

 searches of Wright and Douglas 10 were carried out chiefly by mixing 

 equal volumes of bacteria, serum, and leukocytes (in citrate sus- 

 pension), 11 allowing these elements to remain together at 37.5 C. 

 for varying periods, then staining on slides and determining the 

 degree of phagocytosis by counting the numbers of bacteria taken up 

 by each polynuclear leukocyte. Though many technical difficulties 

 had to be overcome, and although the method at its best still permits 

 of much personal error, careful work and untiring repetition made 

 possible a considerable degree of accuracy, and definite facts regard- 

 ing the mechanism of phagocytosis, heretofore merely suspected, 

 could be recorded. The most important result of these investiga- 

 tions was the unquestionable establishment of the function of serum 

 in the process of phagocytosis, namely, that it in no way "stimu- 

 lated" the leukocytes in the sense of Metchnikoff, but rather acted 

 entirely upon the bacteria, preparing them for ingestion. For this 

 reason Wright coined the word "opsonins" (6^oi>o= I prepare food) 

 for the serum constituents which brought about this effect, believing 

 them to be new antibodies, entirely distinct from the other serum 

 antibodies heretofore recognized. 



Wright and his followers now concluded that the role of the 

 leukocyte in taking up bacteria was entirely dependent upon the 

 opsonin contents of the serum. In a menstruum containing no 

 serum, or in a serum in which the opsonins had been destroyed by 

 heat, they found practically no phagocytic action on the part of 

 washed serum-free leukocytes, and they, therefore, doubted the oc- 

 currence of spontaneous phagocytosis on the part of leukocytes 

 themselves. 



8 Leischman. Brit. Med. Jour., Vol. 2, 1901, and Vol. 1, 1902. 



9 Wrig-ht and Windsor. Jour, of Hyy., Vol. 2, 1902. 



10 Wright and Douglas. Proc. Roy. Soc., 72, 1903, 73 and 74, 1904. 

 See also Wright, "Studien fiber Immunisierung," Fischer, Jena, 1909. 



11 At first bacteria were merely mixed in equal volumes with citrated 

 whole blood. 



