Traumatic brain injury (TBI) is a major cause of disability and a risk factor for early-onset dementia. The injury is characterized by a physical insult followed acutely by complement driven neuroinflammation. Complement, a part of the innate immune system that functions both in the brain and throughout the body, enhances the body’s ability to fight pathogens, promote inflammation and clear damaged cells. Complement plays a role in the brain, regardless of infection or injury, as it influences brain development and synapse formation.