Protein A is a surface protein of S. aureus which binds IgG molecules by their Fc region. In serum, the bacteria will bind IgG molecules in the wrong orientation on their surface, which hinders opsonization and phagocytosis. Mutants of S. aureus lacking protein A are more efficiently phagocytosed in vitro and mutants in infection models have diminished virulence. Due to its affinity for the Fc region of many mammalian immunoglobulins, protein A is considered a universal reagent in biochemistry and immunology.
