Binding of C4 to human IgG and IgM in the IVIG preparations. Inhibition experiments for C4 binding to coated aIgG were performed as described for Fig 1. Percents of inhibition of C4 binding to the aIgG coat at a dose of 2.8 mg/mL IVIG are given in the top panel (░, representative experiment of three with similar results). The supernatants of the incubation mixtures were then removed and incubated on a plate coated with goat anti-C4 antibody. Human IgG (▪) and IgM (□) bound to this anti-C4 coat were then shown by MoAb and goat anti-mouse HRP conjugate. The OD 415 values (averages of duplicates) of the latter assay are given in the bottom panel.