Recombinant antibodies in single-domain format (VHHs) have been recently used for stabilizing antigens during their purification and crystallization. VHHs are also known for their structural stability, and a significant part of them share the characteristic of remaining functionally folded also in the absence of the internal disulfide bond. Therefore, they can be expressed as intrabodies in cell cytoplasm as well as in the bacterial periplasm. This evidence means that, in theory, VHHs can be co-expressed with their antigens independently on the redox constrains. It has also suggested the idea of using co-expression and co-purification of antigen-antibody complexes for maximizing the stabilizing effect of the antibody on its antigen during all the production steps for both cytoplasmic and periplasmic expression strategies.