868 THE MECHANISM OF PHAGOCYTOSIS 



and Wherry,' Ledingham,^ Sawtchenko,^ Barikine/ Kanai^) : (i) a stage in which the 

 particle sticks to the surface, and (2) a stage of ingestion. If phagocytosis were not a 

 process involving a decrease of free energy but rather a process analogous to ingestion 

 by the relatively highly developed organism, the ameba, which probably involves 

 definite energy expenditure (as does any feeding reaction in the higher organisms), 

 then it might be admissible to distinguish an initial stage of adherence between cell 

 and particle. There seems to be no theoretical advantage in making this separation, 

 however, and there is a definite theoretical advantage in considering them two parts 

 of the same process. Stickiness is certainly a matter of surface tension, an approach to 

 an equilibrium; it cannot, for example, demand any extra oxygen consumption on the 

 part of the cell. Phagocytosis in some cases at least is also such a process. Hence they 

 are best regarded as two phases of the same thing. De Haan* has expressed a similar 

 view, believing that phagocytosis ?s a matter of equilibrium only, because it can take 

 place quite independent of ameboid movement, as he has shown. 



CHANCES OF COLLISION AS A FACTOR IN PHAGOCYTOSIS 



Measured differences in rates of phagocytosis may often be attributed to differ- 

 ences in the mechanism of phagocytosis under two given conditions when in reality 

 they are due to differences in the availability of the particles or bacteria to be in- 

 gested. The availability of particles in suspension may be measured by their chances 

 of collision with the leukocytes. It has been shown^ that this depends upon two 

 factors: (i) the relative speed of settling of the particles, Vp, with reference to that 

 of the leukocytes, Vc (this shows, for example, the number of particles per minute 

 which a cell is able to overtake, or vice versa, as it settles) ; (2) a target factor given by 

 the square of the sum of the diameters of the particle. Dp, and the leukocyte, Dc. This 

 factor is proportional to the circular area concentric with the particle, which the 

 center of the leukocyte would have to hit in order that the edge of the leukocyte 

 should not miss the edge of the particle, when one overtakes the other in settling 

 through the solution. The chance of collision, R, is therefore 



(Vp-Vc) {Dp+Dcy = R. 



It has been possible to obtain a rough experimental verification of this formula by 

 measuring the relative speeds of ingestion of different sizes of quartz and carbon 

 particles, the measured rates being what was to be expected from their calculated 

 chances of collision with the leukocytes. Further verification of the formula was ob- 

 tained by rotation of the tubes containing the particles and cells at different speeds 

 without stirring. It was found that the phagocytosis was very low at high speeds be- 

 cause neither particles nor cells had time enough in one revolution of the tube to 



' Kite, G. L., and Wherry, W. B.: /. Infect. Dis., 16, 109. 1915. 



^ Ledingham, J. C. G.: loc. cit. 



3 Sawtchenko, I. G.: Arch, des sc. biol., 15, 145. 1910; 16, 161. 1911; 17, 128. 1912-13. 



tBarikine, W.: Ztschr.f. Immimitdtsforsch. it. e.xper. Thcrap., 8, 72. 1910-11. 



s Kanai, T.: Pfliiger's Arch.f. d. ges. Physiol., 198, 401. 1923. 



'de Haan, G.: loc. cit. ~> Fenn, W. O.: op. cit., 3, 439. 1921. 



