C5 and COVID-19: In order to better understand the importance and role of C5a/C5aR1 signaling on the pathophysiology of COVID-19, we moved to a well-established preclinical mouse model used to study this disease, the K18-hACE2 Tg mice (Tg mice) infected with SARS-CoV-2 (37, 38) (Figure 3A).