Phage-displayed single domain antibodies as recognition elements

The variable domains of antibodies can be expressed as a fusion with pIII, a minor coat protein of the bateriophage M13, for the generation of phage-display antibody reagents. The phage-display system is routinely used to enrich for recombinant antibodies against a specific target antigen from highly diverse naïve and immune libraries. Often once binders are selected, they are expressed as soluble proteins; however, it can be advantageous to use the phage-displayed antibody fragment as a reagent in binding assays. The repeating subunits of the viral capsid allows for significant signal amplification of binding events in downstream assays when utilizing a reporter-conjugated secondary antibody specific for the M13 capsid. Alternatively, labeling of the viral capsid with dyes or biotin molecules provides additional methods of achieving signal amplification in a variety of assay formats. The following protocols detail the use of phage-displayed single domain antibodies in sandwich assays for antigen detection.