CD4 and COVID-19: Another study (Grifoni et al., 2020)also identified circulating SARS-CoV-2 specific CD8+ T cells and CD4+ T cells in COVID-19 recovered patients and also found that CD4+ T cells were positively correlated with the size of anti-SARS-CoV-2 IgG and IgA titers, and anti-acute RBD antibody responses made by COVID-19 patients were positively correlated with the size of specific CD4+ T and CD8+ T cell responses (Figure 2).