Protein A from Staphylococcus aureus has a very high affinity for the Fc regions immunoglobulin G molecules.

It is commonly used in antibody purification processes, and enzyme conjugates can be employed to detect antibodies in immunoassays.

Purified protein A can also be used to coat immunoassay plates in order to orient antibody molecules via their Fc domains. In this orientation both binding arms of the antibody are available for interactions with the antigen.

It may also be immobilized on a solid support such as CNBr-activated agarose for the purification of either monoclonal or polyclonal antibodies.