Figure 2. Depletion strategy for selecting specific bioactive epitopes. In this strategy, a library of candidates is applied to immobilized antigen in the presence of a competitor that binds at the active site of interest. The competitor will prevent the binding of displayed candidates with similar binding characteristics, while allowing undesired candidates to bind. Unbound candidates in solution can be collected and applied to immobilized antigen, this time without the competitor. Such a strategy can facilitate the discovery of therapeutic candidates with exquisite specificity that would not be feasible using in vivo methods.