Protein A is a 40-60 kDa surface protein originally found in the cell wall of the bacteria Staphylococcus aureus (SPA). SPA binds proteins from many of mammalian species, most notably IgGs, and helps inhibit phagocytic engulfment and acts as an immunological disguise via this type of interaction, thus the bacteria will disrupts opsonization and phagocytosis. It binds with the Fc region of immunoglobulins through interaction with the heavy chain. It couples to a wide variety of reporter molecules including fluorescent dyes, enzyme markers, biotin, colloidal gold and radioactive iodine without affecting the antibody binding site. Recombinant Protein A was developed to increase the specificity of the molecule for IgG and is widely used both in research and bioprocessing.