If a new influenza strain suddenly appears, why does vaccine-induced B-cell affinity maturation lead to cross-protection, even though the vaccine was designed for a different strain?

💡 Explanation

Affinity maturation causes higher-affinity antibodies to be produced through somatic hypermutation and clonal selection, thus leading to increased cross-reactivity against related influenza strains, because the antibodies evolve to bind more effectively to conserved epitopes. Therefore, cross-protection arises from this improved binding, rather than original antigenic sin or a suppressed T-cell response.

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