RESUMEN
Phage display is an in vitro technique used in the discovery of monoclonal antibodies that has been used successfully in the discovery of both camelid VHH and shark variable new antigen receptor domains (VNAR). Bovines also contain a unique "ultralong CDRH3" with a conserved structural motif, comprising a knob domain and ß-stalk. When removed from the antibody scaffold, either the entire ultralong CDRH3 or the knob domain alone, is typically capable of binding an antigen, to produce antibody fragments that are smaller than both VHH and VNAR. By extracting immune material from bovine animals and specifically amplifying knob domain DNA sequences by PCR, knob domain sequences can be cloned into a phagemid vector producing knob domain phage libraries. Target-specific knob domains can be enriched by panning the libraries against an antigen of interest. Phage display of knob domains exploits the link between phage genotype and phenotype and could prove to be a high throughput method to discover target-specific knob domains, helping to explore the pharmacological properties of this unique antibody fragment.