It is encoded by the spa gene and its regulation is controlled by DNA topology, cellular osmolarity, and a two-component system called ArlS-ArlR.

Through these interactions in serum, where IgG molecules are bound in the wrong orientation (in relation to normal antibody function), the bacteria disrupts opsonization and phagocytosis.

[4] In 1958, Jensen confirmed Verwey's finding and showed that rabbit pre-immunization sera as well as normal human sera bound to the active component in the staphylococcus extract; he designated this component Antigen A (because it was found in fraction A of the extract) but thought it was a polysaccharide.

[15]) Recombinant versions of protein A also contain the five homologous antibody binding domains but may vary in other parts of the structure in order to facilitate coupling to porous substrates.

[17] Engineered versions are multimers (typically tetramers, pentamers or hexamers) of a single domain which has been modified to improve usability in industrial applications.

Protein A is often coupled to other molecules such as a fluorescent dye, enzymes, biotin, colloidal gold or radioactive iodine without affecting the antibody binding site.

[20] Today, chromatographic separation using protein A immobilized on porous substrates is the most widely established method for purifying monoclonal antibodies (mAbs) from harvest cell culture supernatant.

[21] The choice of protein A as the preferred method is due to the high purity and yield which are easily and reliably achieved.