Antibody Fab fragments or similar binding entities may be combined artificially to give agents with novel, combined specificity, plus enhanced affinity and selective power.

Reminiscent in a limited way with the principle of the sandwich assay, antibody Fab fragments or similar binding entities may be combined artificially to yield agents having enhanced affinity because of their polyvalency, but also a new combined specificity for the totality of target epitopes, and therefore huge opportunities for development of new diagnostic agents. Flexible nucleic acid linkers between binding entities appear to offer advantages by allowing binding to occur without strain.