In some models of macrophage activation it can be triggered by the binding of immune complexes to Fc receptors. But in reality free antibodies can also bind FcRs (that's why we use the Fc block in FACS). Is there a mechanism that can differentiate the binding of a free antibody from binding of the immune complex in terms of FcR-dependent activation? Or does it mean that macrophages can be activated by any free antibodies that might be floating around?