Complement C3 also known as CPAMD1, ASP, C3 is a pivotal protein in the complement system of the innate immune response. Synthesized primarily in the liver, C3 is secreted into the bloodstream and resides mainly in the extracellular space, which is crucial for its role in immediate defense against pathogens. The extracellular location allows C3 to interact directly with foreign invaders, leading to their opsonization and the activation of the complement cascade. Upon activation, C3 is cleaved into C3a and C3b fragments; C3a acts as an anaphylatoxin mediating inflammatory responses, while C3b binds to pathogen surfaces to enhance phagocytosis by immune cells. Dysregulation of C3 can result in increased susceptibility to infections or autoimmune disorders.