Complement system regulators are a group of proteins that regulate the activity of the complement system, a part of the immune system that helps to clear pathogens and damaged cells from the body. The complement system is activated by a cascade of proteolytic enzymes that generate a series of small peptides called complement components. These components can act as opsonins, which mark pathogens for phagocytosis by immune cells, or as anaphylatoxins, which recruit immune cells to the site of infection or injury.