This study looked at a specific immune process in the body, known as the complement system. This mechanism allows microbes and damaged cells to be removed from the body, attacks pathogens, and promotes inflammation. This system is more active in patients with COVID-19 and blocking the system has been considered as a potential treatment. Blood samples from 682 hospitalised patients, including those from the ISARIC4C study, were compared to healthy individuals and multiple components of the complement system were shown to be altered. The levels of some of these markers, known as Ba, iC3b, and properdin, were shown to be associated with disease severity and death. Identifying these new biomarkers helps us to understand how the body responds to COVID-19 and allows new targets for treatment strategies.