C1S and COVID-19: Clusters A to C contain proteins with lower levels in COVID-19 convalescents, e.g. proteins functioning in cell adhesion and platelet degranulation (e.g. fibrinogen A (FGA), VWF), and innate immunity/the complement system (e.g. C1R, C1S, C1QA, C1QC, and GGH) (Fig. 4).