PDCD1 and COVID-19: Recently, immunity, endothelial injury, and complement-induced coagulopathy in COVID-19 have been the topic of many investigations, some of which consistently revealed that peripheral CD8+ T cells from patients with COVID-19 express high levels of exhaustion markers, including programmed cell death protein 1 (PD1) and T-cell immunoglobulin mucin-3 (TIM3) (17).