Regulating expression of the IgG Fc RII/RIII ratio modulates inflammatory responses. An effector cell, shown here as a macrophage, expresses both activation and inhibitory FcγRs and is capable of responding to IC or cytotoxic antibody stimulation at a defined threshold. Upregulation of the activation FcγRIII, induced by IFN-γ or C5a, results in a lowered threshold for IC stimulation and consequently an enhanced inflammatory response. Conversely, upregulation of the inhibitory FcγRII molecule by IVIG raises the threshold for IC stimulation and suppresses inflammatory responses to IgG antibodies.