What are the functions of the complement system?

The main functions of the complement system include: 

1. Opsonization (labeling) and phagocytosis: Two complement proteins, C3b and C4b, activate phagocytosis by binding to foreign particles that invade the body, labeling them for destruction. 
2. Cell lysis: Complement components C5b, C6, C7, C8, and C9 work together to form a membrane attack complex (MAC), which ruptures the microbial cell wall, leading to cell lysis. 
3. Chemotaxis: Complement fragments C5a and C3a, guide neutrophils and macrophages, two types of immune cells, to the site where the antigen is present. This guided movement to the site of inflammation or chemotaxis helps immune cells reach and respond to the source of infection. 
4. Agglutination: Complement proteins cause clustering and binding of pathogens, hindering their movement and making them easier for the immune system to recognize and eliminate.