BTLA and COVID-19: The exhausted T-cell phenotype and upregulation of inhibitory receptors (IRs), including programmed cell-death 1 (PD1) [25], hepatitis A virus cellular receptor 2 (TIM3) [26], lymphocyte-activation gene 3 (LAG3) [25], cytotoxic T-lymphocyte associated protein 4 (CTLA4) [27], and B- and T-lymphocyte-associated protein (BTLA) [28], are often observed in patients with COVID-19, particularly in those with severe disease [29, 30].