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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.

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    $\begingroup$ Keep reading further in that section: ”Many low affinity interactions are formed between receptor and antibody that work together to tightly bind the antibody-coated microbe. 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.” Also keep in mind that soluble antibodies are typically present only in response to an infection. $\endgroup$
    – canadianer
    Feb 19, 2021 at 1:45
  • $\begingroup$ There are many related questions you can look at here. Bear in mind that simply phagocytosing an antigen is different from "activating an immune response". The antibody might be expressed in the membrane of a B cell, bind to a mast cell by its Fc region, or be phagocytosed after opsonization, or cause complement fixation, all with different effects. But note a cell that engulfs unbound antibody cannot present the antigen it didn't bind to. :) $\endgroup$
    – Mike Serfas
    Feb 19, 2021 at 2:32
  • $\begingroup$ @canadianer Ii had seen that, but it doesn't seem to have any explaining power re. differentiating between bound and unbound ab's. See my edit. Thanks. $\endgroup$
    – Reinstate Monica
    Feb 19, 2021 at 14:12
  • $\begingroup$ @MikeSerfas you're right that there is not antigen to present, but it should still degrade the efficiency of the system by hogging or wasting FcR interactions. $\endgroup$
    – Reinstate Monica
    Feb 19, 2021 at 14:14

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