CD4 and infection: However, there was no significant difference in frequencies of SARS-CoV-2 spike-specific and RBD-specific CXCR5- memory CD4 T cells (non-cTfh cells) between severe and mild COVID-19 patients during early infection (Fig. 5d), suggesting that the delayed appearance of virus-specific memory CD4 T cells was restricted to the cTfh cell compartment.