  The reversible aggregation of red blood cells (RBCs) is a process of
erythrocytes clumping that strongly influences the rheological properties of
blood. The adhesion of RBCs has been studied extensively in the frame of
cell-to-cell interaction induced by dextran macromolecules, whereas the data is
lacking for native plasma solution. We apply optical tweezers to investigate
the induced adhesion of RBCs in plasma and in dextran solution. Two hypotheses
'cross-bridges' and 'depletion layer' are being utilized to describe the
mechanism of cells interaction, while both need to be confirmed experimentally.
The results show that in dextran solution the interaction of adhering RBCs
agrees well with the quantitative predictions obtained on the basis of a
depletion-induced cells adhesion model, whereas a migrating 'cross-bridge'
model is more appropriate for plasma. Furthermore, the 'cross-bridge' mechanism
is confirmed by direct visualization of red blood cells adhesion utilizing
scanning electron microscopy (SEM).
