When patients are infected with COVID-19, a cytokine storm (releasing a large amount of several proinflammatory cytokines) often occurs [44] and is followed by lymphopenia, functional impairment of CD4+ T cells, and a quantitative decrease in monocytes, eosinophils, and especially, CD3+ and CD8+ T cells [37, 41]. Here, CD4 is linked to lymphopenia.