A common strategy to promote cell adhesion to biomaterials employs peptide sequences which impart molecular level specificity to cell binding. As many cells present similar surface molecules, however, molecular-level specificity does not always translate to cell-level specificity. The current contribution demonstrates the power of control over the placement of binding groups on a surface to achieve selectivity. Biomaterials are being developed to adhere /kill bacteria while maintaining compatibility with mammalian cells. A key principle being exploited is a cell-level adhesion threshold which develops when surface repulsions are added to molecular-scale attractions between the surface and targets. This talk demonstrates how this threshold arises from the fine tuning of interfacial forces. Different thresholds for different cells allows materials to be designed to sharply discriminate targets.