A study identifies 142 SARS-CoV-2-specific CD4+ T cell epitopes in unexposed humans, revealing preexisting cross-reactive T cell memory against common cold coronaviruses (HCoVs) such as HCoV-OC43, HCoV-HKU1, HCoV-NL63, and HCoV-229E. These HCoVs share partial sequence homology with SARS-CoV-2 and are widely circulating in the general population. The study used blood samples collected before the SARS-CoV-2 outbreak to map these epitopes, demonstrating that CD4+ T cells in unexposed individuals can recognize SARS-CoV-2 peptides with cross-reactivity to HCoVs. The results suggest that preexisting cross-reactive T cell immunity to SARS-CoV-2 may explain some of the variability in COVID-19 disease outcomes. The study also shows that these T cells are largely canonical memory CD4+ T cells, which are typically involved in immune responses to common cold coronaviruses. The findings indicate that cross-reactive T cell responses to SARS-CoV-2 may be more widespread than previously thought, with implications for vaccine development and understanding of immune responses to coronaviruses. The study provides direct evidence that CD4+ T cells can cross-react with homologous sequences from HCoVs, suggesting that preexisting cross-reactive T cell memory may contribute to differences in clinical outcomes among COVID-19 patients. The study also highlights the importance of considering cross-reactive T cell responses in the development of vaccines and immunotherapies for SARS-CoV-2.A study identifies 142 SARS-CoV-2-specific CD4+ T cell epitopes in unexposed humans, revealing preexisting cross-reactive T cell memory against common cold coronaviruses (HCoVs) such as HCoV-OC43, HCoV-HKU1, HCoV-NL63, and HCoV-229E. These HCoVs share partial sequence homology with SARS-CoV-2 and are widely circulating in the general population. The study used blood samples collected before the SARS-CoV-2 outbreak to map these epitopes, demonstrating that CD4+ T cells in unexposed individuals can recognize SARS-CoV-2 peptides with cross-reactivity to HCoVs. The results suggest that preexisting cross-reactive T cell immunity to SARS-CoV-2 may explain some of the variability in COVID-19 disease outcomes. The study also shows that these T cells are largely canonical memory CD4+ T cells, which are typically involved in immune responses to common cold coronaviruses. The findings indicate that cross-reactive T cell responses to SARS-CoV-2 may be more widespread than previously thought, with implications for vaccine development and understanding of immune responses to coronaviruses. The study provides direct evidence that CD4+ T cells can cross-react with homologous sequences from HCoVs, suggesting that preexisting cross-reactive T cell memory may contribute to differences in clinical outcomes among COVID-19 patients. The study also highlights the importance of considering cross-reactive T cell responses in the development of vaccines and immunotherapies for SARS-CoV-2.