  We theoretically investigate the efficiency of initial binding between a
receptor-coated sphere and a ligand-coated wall in linear shear flow. The mean
first passage time for binding decreases monotonically with increasing shear
rate. Above a saturation threshold of the order of a few 100 receptor patches,
the binding efficiency is enhanced only weakly by increasing their number and
size, but strongly by increasing their height. This explains why white blood
cells in the blood flow adhere through receptor patches localized to the tips
of microvilli, and why malaria-infected red blood cells form elevated receptor
patches (knobs).
