When an antibody is bound to an antigen, it can then stimulate a FcR receptor on a phagocyte etc. to respond to the threat. What stops 'free' antibodies from spuriously activating an immune response?
Even if an unbound antibody won't elicit a response, it can still hog the FcRs of expressing cells or cluster around them and degrade the immune system efficiency.
This article seems to imply that being bound to an antigen causes some change in orientation: "their Fc regions point outwards, in direct reach of phagocytes" but I don't really see how its bound state causes that any more than random movement.
It also states
The low individual affinity prevents Fc receptors from binding antibodies in the absence of antigen, and therefore reduces the chance of immune cell activation in the absence of infection
but I fail to understand the logic in that; let there be low affinity, but as much as a bound ab is somehow able to activate the FcR so can an unbound one, unless the bound/unbound state itself affects the affinity, something the article does not explain.