Protein A is a family of affinity chromatography media for the purification of immunoglobulins. Its purification power has been well documented in various antibody purification applications, such as isolation and purification of classes, subclasses and fragments of immunoglobulins from biological fluids and from cell culture media.
The potential applications of protein G include practically all of the current and projected applications of protein A. Protein G and protein A, however, have different IgG binding specificities, dependent on the origin of the IgG. Compared to protein A, protein G binds more strongly to polyclonal IgG, for example, from cow, sheep and horse. Furthermore, unlike protein A, protein G binds polyclonal rat IgG, human IgG3 and mouse IgG1.