Complement Activation and Immune Complex Disease

The complement system is a cascade of plasma proteins that, once triggered, amplifies inflammation, opsonises targets, and can lyse cells. In rheumatic disease, antigen-antibody (immune) complexes activate complement; when complexes deposit in tissues such as the kidney, the resulting complement-driven inflammation causes injury, the hallmark of immune-complex disease.

Definition

Complement activation is the sequential proteolytic cascade — through classical, lectin, and alternative pathways — that generates inflammatory and membrane-attack effectors; immune-complex disease is tissue injury caused by deposition of antigen-antibody complexes that activate this cascade.

Scope

The topic covers how complement is activated, how immune complexes form and deposit, and how this process contributes to systemic autoimmune disease, especially lupus. It also notes the regulators that normally restrain complement. It treats complement and immune complexes as mechanisms, not as a guide to interpreting complement levels in an individual.

Core questions

• What are the three activation pathways of complement and how are they triggered?
• How do immune complexes form and where do they deposit?
• How does complement activation translate complex deposition into tissue injury?
• How is complement normally regulated to protect host tissue?

Key concepts

• Classical, lectin, and alternative pathways
• C3 convertase and the central role of C3
• Membrane attack complex (C5b-9)
• Anaphylatoxins (C3a, C5a)
• Immune-complex (type III) hypersensitivity
• Complement consumption and low C3/C4
• Complement regulatory proteins

Mechanisms

Complement can be activated by antibody-bound antigen (classical pathway), by carbohydrate recognition (lectin pathway), or by spontaneous turnover amplified on surfaces (alternative pathway); all converge on C3 cleavage and formation of the membrane attack complex, as reviewed by Ricklin et al. (2010). When circulating antigen-antibody complexes deposit in vascular beds and tissues, they fix complement locally, releasing anaphylatoxins that recruit neutrophils and amplify inflammation — the basis of immune-complex disease. Active consumption lowers serum complement, which is why falling C3 and C4 accompany active immune-complex disease such as lupus nephritis (Tsokos, 2011). Regulatory proteins normally protect host cells by limiting convertase activity, and their failure shifts the balance toward injury (Zipfel & Skerka, 2009).

Clinical relevance

Complement biology explains why immune-complex deposition damages organs such as the kidney in lupus and why complement proteins are studied as markers of disease activity. This entry describes the mechanism and its general associations; it is not a guide to interpreting complement assays or managing disease in an individual.

Epidemiology

Immune-complex-mediated injury is a central feature of systemic lupus erythematosus and contributes to several vasculitides and glomerulonephritides; the burden of complement-mediated kidney involvement tracks the populations affected by these diseases, as summarised in the cited reviews.

History

Complement was identified at the turn of the twentieth century as a heat-labile serum activity that complemented antibody in killing microbes. The classical, alternative, and lectin pathways were elucidated across the twentieth century, and the concept of immune-complex (type III) hypersensitivity tied antibody-antigen deposition to tissue damage, giving rheumatology a mechanistic account of diseases like lupus nephritis.

Key figures

• John Lambris
• Daniel Ricklin
• Peter Zipfel
• George Tsokos

Seminal works

• ricklin-2010
• zipfel-skerka-2009
• tsokos-2011

Frequently asked questions

Why do complement levels fall in active immune-complex disease?

Active formation and deposition of immune complexes consume complement proteins faster than they are replaced, so serum C3 and C4 decline during active disease and tend to recover as inflammation subsides.

Is complement helpful or harmful?

Both. Complement is essential for clearing pathogens and immune complexes, but when over-activated or poorly regulated it amplifies inflammation and contributes to tissue injury, illustrating its double-edged role in autoimmunity.