A new study published in the journal Science has revealed a possible for why some COVID-19 patients develop long-lasting symptoms after recovering from the infection. The researchers found that these patients had a group of proteins in their blood that indicated a persistent activation of the complement system, a part of the immune system that helps infections. The complement system normally helps clear pathogens from the body, but when it remains active without a target, it can damage healthy cells and tissues. This could lead to reduced oxygen and nutrient to various organs, causing symptoms such as fatigue, brain fog, chest pain, and stomach issues. These symptoms, known as long COVID, affect around 14% of adults who had COVID-19, according to a U.S. survey. The study, which involved 113 COVID-19 patients and 39 healthy controls, found that 40 patients had developed long COVID after six months. The study’s authors also suggest that the complement system could be a potential strategy to prevent long COVID in the first place. They recommend further research to explore the role of the complement system in COVID-19 and its long-term consequences.