CD4 and COVID-19: Our results revealed that the clusters linked to HD exosomes correlated with complement activity and immune response activation, those linked to SEVERE COVID-19 patient exosomes correlated with immune/inflammatory responses and protein metabolism, while those mainly related to MILD COVID-19 patient exosomes were related to immune response, cell growth, signal transduction, and MHC class II receptor functionality (Figure 4, right), well confirming our in vitro results that depicted MILD COVID-19 patient exosomes as strong stimulators of CD4+ T-cell activation and growth.