54 55 SARS-CoV-2, the pandemic coronavirus that causes COVID-19, has infected millions worldwide, 56 causing unparalleled social and economic disruptions. COVID-19 results in higher pathogenicity and 57 mortality in the elderly compared to children. Examining baseline SARS-CoV-2 cross-reactive 58 coronavirus immunological responses, induced by circulating human coronaviruses, is critical to 59 understand such divergent clinical outcomes. The cross-reactivity of coronavirus antibody responses 60 of healthy children (n=89), adults (n=98), elderly (n=57), and COVID-19 patients (n=19) were 61 analysed by systems serology. While moderate levels of cross-reactive SARS-CoV-2 IgG, IgM, and 62 IgA were detected in healthy individuals, we identified serological signatures associated with SARS-63 CoV-2 antigen-specific Fcγ receptor binding, which accurately distinguished COVID-19 patients 64 from healthy individuals and suggested that SARS-CoV-2 induces qualitative changes to antibody Fc 65 upon infection, enhancing Fcγ receptor engagement. Vastly different serological signatures were 66 observed between healthy children and elderly, with markedly higher cross-reactive SARS-CoV-2 67 IgA and IgG observed in elderly, whereas children displayed elevated SARS-CoV-2 IgM, including 68receptor binding domain-specific IgM with higher avidity. These results suggest that less-experienced 69 humoral immunity associated with higher IgM, as observed in children, may have the potential to 70 induce more potent antibodies upon SARS-CoV-2 infection. These key insights will inform COVID-