C4A and COVID-19: In addition, pathway enrichment analysis demonstrates that C4 has higher activity in oxidative phosphorylation, proton transmembrane transport, energy metabolism, oxidative stress, and MHC class II antigen processing and presentation pathways (Fig. 6k), suggesting that the C4 subpopulation may be highly and aberrantly activated32 and thus play a key role in the process of COVID-19.