The physiological function of many cells is dependent on their ability to adhere via receptors to
ligand-coated surfaces under fluid flow. We have developed a model experimental system to measure cell adhesion as a function of cell and surface chemistry and fluid flow. Using a parallel-plate flow chamber, we measured the binding of rat basophilic
leukemia cells preincubated with anti-dinitrophenol
IgE antibody to
polyacrylamide gels covalently derivatized with
2,4-dinitrophenol. The rat basophilic
leukemia cells' binding behavior is binary: cells are either adherent or continue to travel at their hydrodynamic velocity, and the transition between these two states is abrupt. The spatial location of adherent cells shows cells can adhere many cell diameters down the length of the gel, suggesting that adhesion is a probabilistic process. The majority of experiments were performed in the excess
ligand limit in which adhesion depends strongly on the number of receptors but weakly on
ligand density. Only 5-fold changes in
IgE surface density or in shear rate were necessary to change adhesion from complete to indistinguishable from negative control. Adhesion showed a hyperbolic dependence on shear rate. By performing experiments with two
IgE-
antigen configurations in which the kinetic rates of receptor-
ligand binding are different, we demonstrate that the forward rate of reaction of the receptor-
ligand pair is more important than its thermodynamic affinity in the regulation of binding under hydrodynamic flow. In fact, adhesion increases with increasing receptor-
ligand reaction rate or decreasing shear rate, and scales with a single dimensionless parameter which compares the relative rates of reaction to fluid shear.