PDCD1 and lymphopenia: Lymphopenia in COVID-19 is particularly related to the loss of CD4+ and CD8+ T cells (18), which can be caused by 1) overproduction of proinflammatory cytokines IL-6 (3); 2) overexpression of T cell exhaustion markers programmed cell death protein 1 (PD-1) and T cell immunoglobulin and mucin domain 3 (TIM-3) (19); 3) presence of a suppressive neutrophil subset known as granulocytic myeloid-derived suppressor cells (G-MDSC) and its production of alarmin S100A8 and A9 (20).