Neither the D221N/C575A nor D221N/C309L/N297A/C575A mutants that inhibited hemagglutination so effectively (Fig. 10C) bind FcγRIIIA (Fig. 6 and Table II) and would thus not be expected to interfere with FcγRIIIA-dependent Ab-dependent cellular cytotoxicity toward influenza-infected cells by neutralizing IgG present in Flu-IVIG. Here, FCGR3A is linked to influenza.