PHAGOCYTOSIS 217 



is accomplished by the nuclei, which seem to have a specific 

 affinity for this substance. 



Giant-cell formation may also be considered as the result of 

 chemotaxis, the cells moving toward the attracting particle, 

 and when the particle is larger than the cells they spread out 

 upon its surface, their cytoplasm flowing together because of 

 altered surface tension. The peripheral disposition of the 

 nuclei probably depends on the fact that in ameboid motion the 

 nucleus of the cell plays an entirely passive role, being dragged 

 along by the cytoplasm, and hence it is located most remotely 

 from the attracting particle. Digestion of materials taken into 

 a giant-cell seems to go on as in the individual cells that 

 compose it. 1 



Influence of the Serum on Phagocytosis (Opson^ 

 ins). 2 Phagocytosis of bacteria by leucocytes seems not to be 

 merely a reaction between the leucocytes and the bacteria, as has 

 been assumed by Metchnikoff and his school. Wright and 

 Douglas 3 have demonstrated that certain substances in the blood- 

 serum are necessary to prepare the bacteria for phagocytosis, 

 these substances being termed by them " opsonins." If leuco- 

 cytes are washed free from serum with salt solution and let 

 stand in a test-tube with such bacteria as Streptococcus pyogenes, 

 Staphylococcus pyogenes, B. typhosus, B. coli, B. tuberculosis, 

 and various other organisms, no phagocytosis occurs. If, how- 

 ever, some serum from a normal or an immunized animal is 

 added to the mixture, active phagocytosis soon takes place. 

 This opsonin is susceptible to heat, for if the bacteria are let 

 stand with serum that has been previously heated to 60 for 

 ten minutes, and then placed with the leucocytes, no phagocy- 

 tosis occurs, but if unheated serum is used, the bacteria will be 

 taken up by the leucocytes. These observations have been 

 corroborated and extended by Hektoen and Ruediger. 4 The 

 opsonin acts upon the bacteria rather than upon the leucocytes. 

 Certain salts were found to reduce considerably the degree 

 of opsonic action by acting upon the opsonin itself. What 

 changes the opsonins produce in the bacteria that makes them 

 capable of attack by the leucocytes is unknown. The effect of 

 negatively chemotactic substances (i. e., substances preventing 

 chemotaxis) depends upon their destroying the opsonin, accord- 

 ing to the results obtained by Hektoen. 5 



1 See Faber, Jour, of Path, and Bact., 1893 (1), 349. 



2 See also Immunity against Bacteria, Chap. vi. 

 3 Proc. Koyal Soc., 1903 (72), 357; 1904 (73), 128. 

 * Jonr. of Infectious Diseases, 1905 (2), 128. 



5 Complete re'sume' in the Jour. Amer. Med. Assoc., 1906 (46), 1407. 



