Immobilized or purified antibodies, also termed immunoglobulins, are of major importance both for immunochemical techniques in basic research (e.g. Immunoprecipitation) and for diagnostic applications.
Two of the most successful immobilization and purification techniques based on Protein A, a surface protein of Staphylococcus aureus and Protein G, a recombinant streptococcal Protein. Both have a high affinity and binding specificity towards the Fc region of a broad range of mammalian immunoglobulins (Ig).
Immunoglobulins from different species and different isotypes within a species (IgA, IgG, IgM, IgE, IgD) differ in their affinity to Protein A and G [1,2]. Protein G has a higher affinity for IgGs than Protein A, therefore it can bind IgGs from more species than Protein A. But the affinity for various subclasses differs more for Protein A than for Protein G. This allows to purify isotypically pur IgGs. Refer to Protein A and G affinity towards various species and IgG isotypes for a comprehensive overview.
 Harlow and Lane (eds.) (1988) Antibodies. A Laboratory Manual. Cold Spring Harbor Laboratory, N.Y., 617-618.
 Richmann et al. (1982) The binding of staphylococcaI protein A by the sera of different animal species. J. Immunol. 128:2300.