  Membrane-fluctuation-induced attraction between ligand--receptor sites
binding neighboring membranes is studied using meshless membrane simulations
and the Weil--Farago two-dimensional lattice model. For the adhesion sites
binding two membranes, this entropic interaction is too weak by itself for the
adhesion sites to form a large stable domain. However, it is found that this
attraction is enhanced sufficiently to induce large domains either when the
sites bind three or more neighboring membranes together or have anchors that
harden surrounding membranes. The latter effect is understood by the
Asakura--Oosawa type of effective potential in the depletion theory.
