Answer: Affinity maturation
Affinity maturation is the process that leads to increased affinity of antibodies for a protein antigen as humoral response progresses. B cell maturation in the germinal center is a result of somatic mutation of immunoglobulin genes, which provide the molecular basis for the selective survival of B cells producing the highest affinity antibodies. These B cells are thought to have a competitive advantage when antigen becomes limiting in germinal centers.