Moreover, severe COVID-19 illness can be marked by dysfunctional CD4+ and CD8+ T cell immunophenotypes, featured by a concomitant expression of surface markers including PD-1 (programmed cell death protein-1) and Tim-3 (T-cell immunoglobulin and mucin-domain containing-3), associated with an increased susceptibility to the functional exhaustion of T lymphocytes during the course of viral infections [32,33,34,35,36]. This evidence concerns the gene PDCD1 and viral infectious disease.