C3 and COVID-19: Recently, SARS-CoV-2 spikes or C3 have been detected on the red blood cells of COVID-19 patients [25, 35–37], and an acquired decrease in CR1 and increased deposits of C4 fragments on red blood cells among COVID-19 patients have also been found [26], suggesting that the handling and clearance of immune complexes or complement fragment-coated cell debris may play an important role in the pathophysiology of COVID-19.