Complement System and Activation Pathways

The complement system is a network of plasma and membrane proteins that, once triggered, amplifies through a proteolytic cascade to opsonize microbes, recruit inflammatory cells, and lyse susceptible targets. It is a major humoral arm of innate immunity that also links to and amplifies the adaptive response.

Definition

The complement system is a set of serum and cell-surface proteins that are activated in a cascade by microbial surfaces, antibody-antigen complexes, or spontaneous hydrolysis, converging on cleavage of C3 and C5 to drive opsonization, inflammation, and membrane attack.

Scope

This topic covers the three activation routes (classical, lectin, and alternative pathways), their convergence on C3 and C5 cleavage, the formation of the membrane attack complex, the anaphylatoxins C3a and C5a, and the regulators that protect host cells. It treats complement as a mechanistic topic in innate immunity and is not clinical guidance.

Core questions

• What distinct triggers initiate the classical, lectin, and alternative pathways?
• How do the pathways converge on C3 and C5 convertases?
• How are opsonization, inflammation, and lysis generated as effector outputs?
• How do regulatory proteins protect host tissue from complement-mediated injury?

Key concepts

• Classical pathway
• Lectin (mannose-binding lectin) pathway
• Alternative pathway
• C3 and C5 convertases
• Opsonization (C3b)
• Anaphylatoxins C3a and C5a
• Membrane attack complex (C5b-9)
• Complement regulatory proteins

Mechanisms

Three initiation routes feed a common cascade. The classical pathway is triggered when C1q binds antibody-antigen complexes; the lectin pathway when mannose-binding lectin or ficolins recognize microbial carbohydrate; and the alternative pathway through continuous low-level C3 hydrolysis that is stabilized on microbial surfaces lacking host regulators. All converge to form C3 convertases that cleave C3, depositing C3b for opsonization and generating C5 convertases. C5 cleavage releases the anaphylatoxin C5a and initiates assembly of the membrane attack complex (C5b-9), which can lyse susceptible cells. The anaphylatoxins C3a and C5a recruit and activate leukocytes and amplify inflammation, while host-cell regulators restrict the cascade to protect self tissue.

Clinical relevance

Complement biology underlies understanding of recurrent infections in complement deficiencies, hereditary angioedema, and complement-mediated tissue injury in conditions such as paroxysmal nocturnal hemoglobinuria and certain kidney diseases. This entry describes mechanisms for reference and is not a basis for diagnosis or treatment.

Evidence & guidelines

Descriptions draw on authoritative reviews of complement biology rather than quantitative clinical evidence or practice guidelines.

History

Complement was described at the turn of the twentieth century as a heat-labile serum activity that complemented antibody in lysing bacteria, work associated with Jules Bordet and Paul Ehrlich. The cascade's components, the discrete activation pathways, and the regulatory network were subsequently characterized, establishing complement as a central effector and regulator of innate immunity.

Key figures

• Jules Bordet
• Paul Ehrlich
• Mark Walport
• John Lambris

Seminal works

• walport-2001
• ricklin-2010

Frequently asked questions

What do the three complement pathways have in common?

Although the classical, lectin, and alternative pathways are triggered by different signals, all converge on the cleavage of C3 (and then C5), generating the same core effector functions: opsonization, inflammation through anaphylatoxins, and membrane attack complex formation.

Why doesn't complement destroy the body's own cells?

Host cells express regulatory proteins that accelerate decay of the convertases and inhibit membrane attack complex assembly, so the cascade is normally restrained on self surfaces while proceeding on unprotected microbial surfaces.