Protein A is a 42 kD surface protein originally found in the cell wall of the bacterium Staphylococcus aureus. It is widely used in biochemical research and industry because of its ability to bind immunoglobulins. It is composed of five homologous Ig-binding domains that fold into a three-helix bundle. Each domain is able to bind proteins from many mammalian species, most notably IgGs. It binds the heavy chain within the Fc region of most immunoglobulins. HRP Protein A is useful for detecting primary antibodies, e.g., in ELISA, ELISPOT, IHC, or Western blotting. For IP followed by Western blotting application, HRP protein A will avoid IgG heavy/light chain issue because it only recognizes the IgG in its native form, and the antibody used to IP the target is usually boiled with the immunoprecipitates and gets denatured.