Some model experiments in hemodynamics: VI. Two-body collisions between blood cells

We describe a traveling microtube technique, previously applied to polystyrene latices (11), in which the trajectories of colliding microscopic spheres in Poiseuille flow may be photographed and analyzed with the aid of hydrodynamic theory to calculate the interaction forces due to electrostatic repulsion, van der Waals attraction or polymer bridging which occur when surfaces approach closer than 50 nm. The method was applied to human red cells of antigenic type A or B, sphered at constant volume or surface area, fixed with glutaraldehyde and suspended in aqueous glycerol with or without the addition of the corresponding antibody. The trajectories of transient doublets of colliding sphered red cells exhibited marked asymmetry, not due to electrostatic repulsion but due to the presence of surface asperities on the particles. In the presence of antisera, permanent doublets formed due to bridging of antibody between antigenic sites on adjacent cell surfaces. Forces of the order of 0.1 nN were required to break up these linkages. Measurements of the period of rotation showed that permanent doublets behaved as rigid dumbbells except at the lowest antiserum concentration. Trajectories of colliding rabbit granulocytes were also studied and found to be markedly asymmetric due to the ruffled surfaces of the cells. Nevertheless, some collisions resulted in the formation of permanent doublets.