Display-based selection solves a practical problem: if you have a library of billions of antibody variants, how do you find the few that bind your target, and how do you know which gene made them?
The answer is to keep each variant physically attached to its own genetic instructions. In phage display, each antibody variant is displayed on the coat of a bacteriophage particle, and the gene encoding that variant is packaged inside the same particle. The variant and its blueprint are never separated.
Selection then proceeds in rounds. The library is applied to the target antigen, which has been immobilized on a surface such as the well of a plate. Variants that bind stay attached; variants that do not bind are washed away. The bound phages are eluted, and because they still carry their genes, they can be used to infect bacteria and be amplified into a new, enriched library.
One round rarely produces a pure binder. Each round increases the proportion of target-binding variants, so the process is repeated — typically two to five rounds — until the population is dominated by binders. Stringency can be increased between rounds, for example by lowering the amount of target or adding competing antigen, which favors variants with higher affinity.
After the final round, individual clones are picked and sequenced, and their antibodies are produced and tested. The key conceptual point is that selection does the searching, not the scientist: the experiment applies a binding criterion to an enormous library at once, and only variants that satisfy it survive.