CXCR4 and COVID-19: The majority of CD74+ convCD4+ T cells at all stages of differentiation co-expressed the CXCR4 molecule, but increased frequencies of CXCR4+ CD74+ convCD4+ T cells in COVID-19 were only detected in cells with a naïve phenotype (mean: healthy 83.8%, mild: 82.1%, severe: 93.3%; Figure 4C, lower panel).