C3 and Alzheimer disease: Complement component (C3), an essential immune regulatory factor, plays a crucial role in immune responses, particularly in the elimination of pathogens and the regulation of inflammation.[18] Previous studies have indicated that endogenous C3 protein was excessively activated in the brains of patients with AD and in relevant models, and contributed to the progression of AD pathology and cognitive impairment.[19, 20, 21] Nevertheless, it remains unclear whether peripheral C3 protein could influence the CNS and cognitive function.