KLRD1 and COVID-19: In addition, the elevated level of markers of exhaustion including programmed Cell death protein (PD1) and NKG2A (CD94) on the cytotoxic cells such as CD8+ T cells and NK cells [44,178,179] predicts impairment of the immune system observed in COVID-19 patients since the immune system is shifted toward immunosuppressive Th2 [5].