CD4 and COVID-19: To determine whether COVID-19 plasma exosomes directly regulated the immune response of a specific type of immune cells comprising PBMC, we separated CD4+ T cells, CD8+ T cells, and CD14+ monocytes from PBMCs using MACS MicroBeads (Miltenyi Biotec Inc.), followed by treatment with plasma exosomes and subsequent flow cytometry gating on live cells.