Affinity chromatography with protein A has become the foundation for purification of nearly every therapeutic IgG in commercial production. One of the features most responsible for its success has been its compelling simplicity. IgG binds. Contaminants do not. Load, wash, and elute pure IgG. In the real world, however, protein A does not elute pure IgG. It typically contains several hundred to a few thousand parts per million (ppm) contamination by host-cell proteins (HCPs) and other contaminants. Numerous studies demonstrate…