Most FcγRs (Table 2) are weak, low‐affinity receptors (affinities in the micromolar range) for IgG‐Fc, irrespective of whether the IgG is uncomplexed, monomeric or when it is complexed with antigen (i.e. an immune complex). The very avid binding of immune complexes to an effector cell surface that displays an array of FcγR molecules is the result of the collective contributions of the low‐affinity interactions of each Fc of the IgGs in the complex with an FcγR. This avidity effect is necessary as the FcγRs operate in vivo in environments of high concentrations of uncomplexed monomeric IgG (normally 3–12 g L–1). Thus, the avid multivalent binding of the complex out competes uncomplexed, monomeric IgG. The notable exception to this is the enigmatic FcγRI. This receptor shows high, nanomolar affinity for uncomplexed monomeric IgG and thus would be expected to be constantly occupied in vivo by the normal circulating monomeric IgG. However, IgG dissociation permits engagement with immune complexes. Furthermore, FcγRI is not expressed or expressed poorly on resting cells, requiring interferon‐γ for induction of its expression, presumably at sites of inflammation.