CD4 and COVID-19: In relation to the chronic activation of the immune system that is associated with the five hypotheses mentioned above, COVID-19 patients have shown long-term alterations for up to six months following their discharge in their primary B- and T-cell functions as well as in the vast majority of their CD4+ and CD8+ T-cell subsets, which are critical for the control of intracellular and extracellular pathogenic infections [277].