Patients with B/Yamagata influenza virus had a 2-fold higher median HA-specific FcγRIIIa-binding antibody titer (median titer = 1280 against the B/Phuket HA) than A/H1N1 patients (median titer = 640 against the A/Cali09 HA) following Flu-IVIG treatment, and this may contribute to Flu-IVIG protection in patients with influenza B. Similar to the HAI results, influenza B participants also showed 2- to 4-fold greater FcγRIIIa-binding antibody titers before infusion with Flu-IVIG and after placebo infusion. Here, FCGR3A is linked to influenza.