C2 and COVID-19: At HLA-C, which is expressed at much lower levels than HLA-A and HLA-B and is believed to be more specific for NK cell regulation rather than CTL activation, the distribution of HLA-C1 and HLA-C2 was similar between controls (C2: 15.7%) and severe COVID-19 patients (C2: 18.8%), but the C2 ligand was more frequent in mild COVID-19 patients (22.8%) (Figure 4A).