Free immunoglobulin molecules bind most Fc receptors with very low affinity and can not cross-link Fc receptors. Antigen-bound immunoglobulin, however, can bind effectively to Fc receptors in a high-avidity interaction because several antibody molecules that are bound to the same surface bind to multiple Fc receptors on the surface of the accessory cell. This Fc receptor cross-linking sends a signal to activate (or sometimes inhibit, not shown) the cell bearing it.