The study of Zhou, Chi [8] reported that in patients infected with SARS-CoV-2, the quantity of total T cells and CD4+ and CD8+ T cell subsets were largely suppressed and functionally exhausted, while the control and clearance of pathogens were weakened in patients with diabetes mellitus, causing the inhibition of immune defenses in response to the invasion of COVID-19. Here, CD4 is linked to COVID-19.