Why does the secondary immune response produce IgG quicker than in the primary response?

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Memory B cells that formed during the first exposure are the reason IgG comes in faster the next time. After the initial encounter, some B cells become long-lasting memory cells and have already undergone class switching to IgG and accumulated somatic hypermutations, giving them high affinity for the antigen. When the same antigen appears again, these memory cells are quickly reactivated, proliferate, and differentiate into plasma cells that secrete IgG rapidly and in larger amounts. This rapid, targeted response contrasts with the primary response, where naive B cells must first be activated, undergo clonal expansion, and switch to IgG, taking longer to produce antibodies. The other ideas aren’t correct because they either rely on changes to the antigen that don’t drive isotype switching, assume all B cells default to IgG, or presume IgG genes are already active in all B cells without prior activation.