C3 and COVID-19: Severe COVID-19 patients have high circulating C5a in their blood as well as high levels of processed C3 (Mastellos et al, 2020; Skendros et al, 2022), suggesting that uncontrolled complement activation might be involved in the severity of COVID-19 (Afzali et al, 2022; Posch et al, 2021).