Goat anti-Mouse IgG Fc polyclonal antibody for ELISA, WB, IHC.. Protein A is asurface protein of S.aureus which binds IgG molecules by their Fc region. Inserum, the bacteria will bind IgG molecules in the wrong orientation on theirsurface which hinders opsonization and phagocytosis. Mutants of S. aureus lackingprotein A are more efficiently phagocytosed in vitro, and mutants ininfection models have diminished virulence.Due to its affinity for the Fcregion of many mammalian immunoglobulins, protein A is considered a universalreagent in biochemistry and immunology.