The complement system is a powerful ally in our immune defense. It coats pathogens, making them easier for immune cells to spot and destroy. This process, called opsonization, kicks off a chain reaction that ramps up our body's response to invaders.

But complement doesn't stop there. It can punch holes in some bad guys, causing them to burst. It also helps clean up immune complexes and dead cells, preventing potential harm. Plus, it acts as a bridge between different parts of our immune system, making everything work better together.

Complement System in Immunity

Complement in opsonization and phagocytosis

• Opsonization process coats pathogens with complement proteins enhancing recognition (C3b and C4b primary opsonins)
• Enhanced phagocytosis occurs when phagocytes recognize opsonized pathogens via complement receptors (CR1, CR3, CR4)
• Increased efficiency of pathogen clearance accelerates immune response
• Amplification of immune response recruits more phagocytes to infection site creating positive feedback loop

Complement-mediated cell lysis

• MAC formation process begins when C5 convertase cleaves C5 into C5a and C5b initiating assembly
• Sequential binding of C6, C7, C8, and C9 proteins forms transmembrane pore (10-16 nm diameter)
• Lysis mechanism disrupts cell membrane integrity causing osmotic imbalance leading to cell death
• Target cells include gram-negative bacteria, enveloped viruses, and parasites (malaria)

Complement for immune complex clearance

• C1q binds to antibody-antigen complexes activating classical pathway
• Enhanced solubilization and removal by phagocytes prevents tissue damage
• Apoptotic cell clearance initiated by C1q recognition of altered cell surface molecules
• Mannose-binding lectin (MBL) binds to apoptotic cells promoting opsonization
• Prevention of autoimmunity through rapid removal of potentially harmful cellular debris
• Maintenance of tissue homeostasis by efficient clearance of dead cells

Complement as immune system link

• Enhancement of B cell responses via complement receptors (CD21/CD35) lowers activation threshold
• T cell modulation through complement receptors regulates differentiation and function
• Antigen presentation improved by complement-mediated uptake by antigen-presenting cells
• Cytokine production stimulated by C3a and C5a as anaphylatoxins (histamine release)
• Memory response augmentation involves complement in germinal center reactions
• Enhanced antibody production and affinity maturation strengthen adaptive immunity