C5 and COVID-19: Serum C5a and C5aR levels were increased in COVID-19 patients (n = 82), compared to healthy subjects (n = 10) and C5a/C5aR serum levels positively correlated with disease severity where the most severe COVID-19 patients (n = 28) showed the most increased levels compared to moderate (n = 34) and mild (n = 10) patients [114].