The molecular-level specificity between antibodies and target antigens, or between complimentary pairs of DNA strands does not translate to the cellular level: The surfaces of most cells have a similar makeup of adhesion molecules yet cellular interactions can be highly selective, even specific, leading to targeted adhesion and signaling. Once of the mechanisms by which cellular-level specificity is achieved is via multivalent interactions that are related to adhesion and signaling thresholds. As an example, T-cell binding to a target must exceed a threshold number of bonds before the T-cell becomes activated to kill the target. In this way, T-cells distinguish their targets from other cells including cells of the self. Here we describe an electrostatic –based system in which electrostatic “bonds” between a collecting surface and a target must exceed a threshold number before adhesion takes place. We demonstrate how this criteria leads to extremely sharp selectivity in particle capture, and we demonstrate how the threshold can be tuned through hydrodynamic and background surface forces. We then demonstrate how the same surfaces can be employed as sensors and separation media for cells and bacteria, showing quantitative agreement for the effects of shear and receptor density between the model system and bacterial adhesion to natural substrate.