To determine whether these BnAbs were unique to the subject population tested or are present more broadly, we quantified the anti-H5 and F10-like Abs in a commercial IVIG and determined their breadth of heterosubtypic binding and neutralization activity. The IVIG contained pooled IgGs from thousands of donors and is representative of the pre-immune IgG Ab composition in the general population. Although it cannot be formally ruled out that IVIG is truly H5 naive, this was the best representative sample available for this study. H5-immobilized magnetic beads were used to affinity-purify F10-like and anti-H5 Abs from the IVIG. F10-like Abs were isolated by Bio-F10 competition elution from the total H5-beads bound Abs, and the remaining H5-bound Abs on the H5-beads were released with acid elution. By quantitative ELISA, we found that, from 100 mg of IVIG, 10 lg of Abs bound to the H5-coated plate (0.01% of the total). Of these, 10% could be purified using affinity purification with H5-beads by acid elution. The final yields were 1-1.4 lg acid-eluted anti-H5. The Bio-F10 competitive elution yielded 0.1 lg of F10-like Abs per 100 mg of IVIG (0.001% of the total Ig level) and with similar or higher efficiency of recovery as compared with acid elution.