KLRC1 and COVID-19: Thus, circulating NK cells in COVID-19 patients display an upregulation of the inhibitory receptor NKG2A that is associated with disease severity, consistent with recent results.8 Bronchoalveolar lavage fluid (BALF) analysis showed a lack of CD16+CD57+ mature NK cells in the lungs of ARDS patients (Fig. 1d), suggesting that the decrease in mature NK cell levels observed in blood (Fig. 1b, d) is not a consequence of their migration to infected lungs.