866 THE MECHANISM OF PHAGOCYTOSIS 



tween them becomes less there is a repulsion due to the redistribution of the charges 

 on the outer layers. Ponder has simplified the situation somewhat in considering only 

 the outer layer of the double layer in his calculations. He calculates the mutual redis- 

 tribution of the charges and the resulting electrical forces between the cell and the 

 particle (which is considered small in comparison) on the basis of Coulomb's law, and 

 arrives at the conclusion that when the distance between the two bodies becomes very 

 small the repulsion between them (in case they both carry a charge of the same sign) 

 becomes an attraction. This result is true provided his assumptions are true, and a 

 valuable hypothesis is thus provided in the light of which future experimental results 

 can be analyzed. In discussing this result, Ponder has suggested that the preliminary 

 stage of phagocytosis is that the electrical attraction causes the particle to stick to the 

 cell. "Thereafter the early stage of ingestion takes place followed by more and more 

 complete ingestion until an equilibrium is reached," Since electrical forces are in- 

 cluded in surface tension forces, this distinction seems unwarranted. The whole 

 process of sticking and ingestion is part of a single process, provided it be admitted 

 that phagocytosis is an approach to an equilibrium. This is of course not necessarily 

 true in all cases. 



THE SIZE or THE PARTICLE AS A TEST OF THE THEORY 



According to the theory outlined, the angle of contact between a cell and a solid 

 particle should be independent of the size of the particle. If a small particle is ingested 

 by a certain cell, a large one will be also. If the particle is very large, ingestion is of 

 course impossible and one speaks instead of a "spreading" of the cell on the particle. 

 Tait' has drawn attention to numerous instances in which it is found that particles 

 are ingested only by those cells which spread out on glass. This may be regarded as a 

 confirmation of the theory, for in both cases the contact angle would be small between 

 the cell and the solid. 



Changes in the physico-chemical environment of the cells affect both their 

 phagocytosis and their spreading. If such effects are caused merely by surface tension 

 changes, then a given change in the environment should affect both spreading and 

 phagocytosis in the same direction, provided the nature of the solid is the same in 

 both cases. I have attempted to test the theory from this point of view but have found 

 mostly a lack of parallelism.^ 



Thus it has been found that while leukocytes ingest quartz particles more readily 

 than carbon particles in acid solutions, they do not stick to flat surfaces of glass 

 better than to carbon surfaces in similar solutions. Likewise, the addition of serum 

 to a suspension of cells in 0.9 per cent NaCl increases their ability to phagocytize 

 quartz but decreases their stickiness to glass surfaces. Again the stickiness of leuko- 

 cytes to a glass slide increases with the alkalinity while the phagocytosis of quartz 

 particles by the same cells increases with the acidity. Finally, the optimum tempera- 

 ture for phagocytosis by mammalian leukocytes is 37° C. while the optimum for 

 stickiness to glass is at 20° C. (one to two hours after contact). In the latter case some 

 reason was found for believing that the stickiness was really a maximum at 37° C. but 



' Tait, J.: Quart. J. Exper. Physiol., 13, i. 1918-20. 

 ' Fenn, W. O.: op. ciL, 5, 143 and 169. 1922. 



