What is the role of complement proteins in the immune response?

Complement proteins play a crucial role in the immune response by acting as a non-cellular component that interacts with pathogens. Upon activation, these proteins help mark pathogens for destruction, enhance phagocytosis, and recruit inflammatory cells to the site of infection. They do this through a series of enzymatic reactions known as the complement cascade, which leads to the formation of a membrane attack complex that can directly lyse some pathogens.

The complement system operates alongside antibodies and other components of the immune system, but it does not produce cytokines or function as antibodies itself. It does not directly kill pathogens by injecting toxins nor does it bind to antigens like antibodies do. Instead, its primary function revolves around the recognition and targeting of pathogens, thereby facilitating a more effective immune response.