Background: In 2020, the B/Yamagata lineage of influenza B disappeared from circulation. Understanding the immunological conditions preceding this loss can help explain why the lineage disappeared and has implications for our understanding of viral evolution and ongoing vaccine policies.&nbsp; <div> <br> </div> <div> Methods: We measured neutralising antibody responses in age- and sex-matched blood donor cohorts collected in 2020, 2023, and 2025 (n=114 per cohort) against a panel of pseudotyped influenza B viruses spanning 79 years of evolution, validated against live virus neutralisation assays. We performed antibody pulldown assays using full-length and head domain HA proteins from B/Yamagata (B/Phuket/3073/2013) and B/Victoria (B/Washington/02/2019) lineages, testing purified antibodies against chronologically distinct pseudotyped viruses. We used Bayesian network analysis, and LASSO regression to identify potential molecular determinants of cross-lineage neutralisation which were then confirmed via site-directed mutagenesis, and epitope-specific peptide pulldowns.&nbsp; </div> <div> <br> </div> <div> Findings: In 2020, population immunity was asymmetrically focused on B/Yamagata viruses. Despite no B/Yamagata circulation after 2020, neutralisation of recent B/Yamagata strains increased in 2023 while B/Victoria responses remained unchanged, consistent with antigenic seniority directing recall responses toward B/Yamagata. This trend inverted by 2025, with B/Yamagata immunity declining and B/Victoria immunity increasing. Antigen-specific purified antibodies showed lineage-biased potency, neutralising B/Yamagata viruses more effectively than B/Victoria viruses. We identified the 120-loop of the HA head domain as a critical cross-lineage epitope, with the charge state at a single convergently evolving residue at position 131 determining cross-reactive potency between lineages.&nbsp; <div> <br> </div> <div> Interpretation: B/Yamagata's acquisition of a positively charged residue at position 131, shared with B/Victoria, likely increased its cross-neutralisation by Victoria-raised immunity preceding its disappearance. Although antigenic seniority sustained cross-reactive responses to B/Yamagata after its extinction, the waning of this effect by 2025 suggests that population immunity to B/Yamagata is now declining. This has implications for influenza B vaccine policy. </div> </div>