CLEC2D and COVID-19: On the protein level, NK cells from all participants with COVID-19 exhibited upregulation of human leukocyte antigen (HLA) and death receptors (DR4–5); however, narrow neutralizers expressed significantly greater levels of CD8A and LLT-1 (markers of cytotoxicity and activation, respectively) when compared with broad neutralizers38,39 (Fig. 3g).