A surface force apparatus was used to measure a long-range attractive protein-ligand force at separations D less than 85 angstroms. This force may effectively "steer" ligand trajectories, resulting in a greater than 27-fold enhancement of the association rate. A much stronger specific attraction is measured at contact (D less than 4 angstroms). A sevenfold increase in intermembrane adhesion resulted from increased lateral mobility of the receptors and molecular rearrangements in membranes above the solid-fluid transition temperature.