"Responding cells in unexposed donors were predominantly found in the effector memory CD4+ T cell population (CD45RAnegCCR7neg), followed by the central memory T cells (CD45RAnegCCR7pos) (
30) (
Fig. 3, A, B, and D). Comparable patterns of effector and central memory cells were observed among the antigen-specific CD4+ T cells detected in the COVID-19 cases (
Fig. 3, C and D). In conclusion, the CD4+ T cells in unexposed donors that recognize SARS-CoV-2 epitopes, and epitopes from other HCoVs, have a memory phenotype.."
Basically the protein spikes on COVID are similar to the protein spikes of other corona viruses (common cold). T-Cells recognize infections. Some people who have not been exposed to COVID have T-Cells that recognize COVID and fight it off.
Probably the reason that 80% of the people that are infected have no symptoms or very mild symptoms.