PDCD1 and COVID-19: Lymphopenia is directly linked to COVID-19 death, as inflammatory cytokines are thought to cause lymphocyte apoptosis.90 The frequency of CD4 + and CD8 + T lymphocytes is considerably reduced during COVID-19 infection and patients in severe clinical stage had much lower counts of NK cells, CD14 + monocytes, CD4 + T cells and CD8 + T cells.89 Studies has revealed that the CD95 and programmed cell death protein-1 (PD-1) expression in circulating CD4 + and CD8 + T cells is greater in SARS-COV-2 patients than in healthy persons.