C5AR1 and COVID-19: The expression of CXCL10 was strongly correlated with the levels of anti-RBD IgG and IgA in COVID-19 patients, which is a feature also observed in patients with autoimmune diseases.10 C5a/C5aR1 interactions in CD4+ T cells are associated with an increased percentage of TFH cells and an elevated level of autoantibody production.11 Concordant with these results, we found a strong correlation between the levels of C5a and anti-RBD IgA; however, only a weak correlation was observed between the levels of C5a and anti-RBD IgG (Fig. 1c).