The standard capture purification of full-size, classical antibodies is typically performed with Protein A affinity chromatography. Binding of the Protein A affinity ligand to the “Fc” region of antibodies (Abs) (abbreviations are defined at end of paper) takes place at the juncture of the constant domains 2 and 3 of the Ab heavy chains (Lewis et al. 2008). This high affinity binding primarily involves hydrophobic interactions. Antibodies that belong to the same subclass have greater than 95% homologous Fc-regions, allowing Protein A to be a capture platform for a wide range of antibodies (Ghose et al. 2005).

Following the wave of successful commercial monoclonal antibody products, various forms of antibody fragments are now becoming an important class of next generation therapeutic proteins. This includes Fabs and fusion proteins of the Fab variable domains. From the variable domain of the heavy-chain antibodies of camelids, the VHH sdAbs have been derived. These VHHs represent some of the smallest antigen binding antibody-derived proteins. As such they are more stable than full size mAbs, can be produced in microbial organisms, and offer higher target binding events per gram of product. Due to the lack of an Fc region, these antibody fragments cannot be captured with most engineered Protein A affinity ligands. However, Amsphere™ A3 Protein A ligand exhibits a high affinity for VHH single domain antibodies.