Complement inhibitors are substances that block the activity of the complement system, a part of the immune system that helps the body fight infections. The complement system consists of proteins that work together to identify and destroy pathogens, but sometimes it can become overactive and damage healthy tissues. Complement inhibitors help regulate this system, preventing excessive inflammation and tissue damage. These inhibitors can be naturally occurring in the body or can be developed as medications for certain diseases. For example, eculizumab is a complement inhibitor used to treat conditions like paroxysmal nocturnal hemoglobinuria and aHUS, where the complement system is improperly activated.