Protein A

Protein A is well known for its ability to bind the Fc fragment of most mammalian immunoglobulins especially IgG. This material is a polypeptide surface receptor isolated from the cell membranes of staphylococcus aureaus. It contains four nearly homologous regions and can bind two immunoglobulin molecules. Protein A binds immunoglobulins without interfering with the antigen binding sites which allows the formation of ternary complexes consisting of Protein A, antibody, and antigen. These properties make Protein A useful as a ligand for the affinity purification of monoclonal and polyclonal antibodies and in various immunoassays.