CD8A and COVID-19: Therefore, it is possible that during the recovery period of individuals with critical illness from COVID-19, the increase in the cell numbers of NK cells, CD4+ T cells, and CD8+ T cells could be facilitated by a common lymphoid progenitor in the bone marrow, which gives rise to precursor cells of T cells, NK cells, and innate lymphoid cells (Gasteiger and Rudensky, 2014).