IVIg binds to B-cell surface in a dose-dependent manner. (A) Ramos B cells were incubated with a range of FITC-labeled IVIg for 1 hour at 4°C. Binding of IVIg was determined by FACS and MFIs measured at each point of FITC-IVIg binding. Unconjugated FITC was used as the negative control. (B) Blood B cells (open bars), tonsillar B cells (gray bars) and Ramos B cells (closed bars) were incubated with increasing concentrations of IVIg-FITC and binding measured by FACS.