phage display principle 

The phage display technology, first described over a decade ago, has opened new windows in the production of recombinant antibody fragments with the promise of trapping the naive immune repertoire in vitro. Phage display is a term describing display of foreign (poly) peptides on the surface of phage particle. The technique differs from conventional expression systems, however, in that the foreign gene sequence is spliced into the gene for one of the phage coat proteins, so that the foreign amino acid sequence is genetically fused to the endogenous ones of the coat protein to make a hybrid “fusion” protein. Using recombinant DNA technology, collections of billions of certain ligands (e.g., peptides or proteins, antibody fragments), cDNA-encoded or gene fragment-proteins presented on phage (so called phage display libraries) can be constructed and surveyed for specific affinity or activity (Fig. 1).
https://www.creative-biolabs.com/blog/index.php/fundamental-technologies-for-antibody-production-phage-display/