Complement system

Components and activation pathways

Complement consists of many proteins, often named C1 through C9 and various fragments. Activation proceeds through three main routes: the classical pathway (triggered by antibody–antigen complexes), the lectin pathway (triggered by carbohydrate patterns on microbes), and the alternative pathway (a continuously low-level amplification loop that can escalate on foreign surfaces). All three converge on cleavage of C3, a central hub that amplifies the response and links to downstream effectors.

Mechanisms of action

Activated complement mediates host defense by several complementary mechanisms:

• Opsonization: C3b and related fragments coat microbes to promote uptake by phagocytes.
• Inflammation: Small cleavage products such as C3a and C5a act as chemoattractants and activate inflammatory cells; these are often called anaphylatoxins (cytokine-like mediators).
• Direct lysis: Assembly of the membrane attack complex (MAC) forms transmembrane pores that disrupt the cell membrane of susceptible bacteria and cells.

Regulation and synthesis

Because complement is potently destructive, multiple regulators prevent damage to host tissues. Most complement proteins are produced in the liver, but many cell types can also make components or regulators locally. Proteolytic enzymes called proteases cleave inactive precursors in a controlled cascade, producing effector fragments and amplifying the signal.

Clinical significance and uses

Complement contributes to protection from infection, clearance of immune complexes and apoptotic cells, and shaping adaptive immune responses. Deficiencies or dysregulation can lead to increased susceptibility to recurrent infections, autoimmune diseases, or inflammatory tissue injury. Complement is also a therapeutic target: inhibitors that block specific steps are used to treat some rare and common conditions, illustrating the system's clinical relevance.

History and research directions

The concept of a heat-labile 'complement' that assisted antibodies emerged from experiments in the late 19th and early 20th centuries. Modern research continues to define molecular details, the interplay between complement and cells of the immune system, and how complement participates in noninfectious diseases such as age-related macular degeneration, ischemia reperfusion injury, and neuroinflammation. For introductory resources see phagocyte references, general pathogen overviews at pathogen resources, innate immunity primers at innate immunity, and reviews of complement synthesis and regulation at liver and production, protease mechanisms, inflammatory mediators, cascade models, and membrane attack complex.