Primary Immunodeficiencies: Complement and Phagocytic Cell Disorders

This topic covers inborn errors of innate immunity in which the complement system or phagocytic cells are defective. Complement deficiencies impair the opsonic, lytic, and immune-clearance functions of the cascade, while phagocyte defects such as chronic granulomatous disease and leukocyte adhesion deficiency impair the recruitment and microbial killing functions of neutrophils and macrophages.

Definition

Complement deficiencies are inborn errors affecting one or more proteins of the complement cascade or its regulators, impairing opsonisation, membrane attack, and immune-complex clearance. Phagocytic cell disorders are inborn errors impairing the number, adhesion, migration, or microbicidal activity of neutrophils and mononuclear phagocytes, the prototype being chronic granulomatous disease, in which the respiratory burst that kills ingested microbes is defective.

Scope

The entry surveys the complement and phagocyte categories of the international classification of inborn errors of immunity: deficiencies of early, terminal, and regulatory complement components, and defects of phagocyte number, adhesion, and oxidative killing. It frames how each defect maps to a characteristic susceptibility pattern, as a reference rather than a diagnostic or treatment protocol.

Core questions

• Which arm of innate immunity is defective: complement or phagocyte function?
• How does the level of a complement defect (early, terminal, or regulatory) predict its clinical association?
• Why do phagocyte killing defects produce infection with catalase-positive organisms and granuloma formation?

Key concepts

• Complement cascade (classical, alternative, lectin pathways)
• Early-component complement deficiency
• Terminal-component (C5-C9) deficiency and Neisseria susceptibility
• Complement regulatory protein defects
• Chronic granulomatous disease (NADPH oxidase defect)
• Respiratory (oxidative) burst
• Leukocyte adhesion deficiency
• Granuloma formation
• Catalase-positive organisms

Mechanisms

Complement is an enzymatic cascade that opsonises microbes, assembles the lytic membrane attack complex, and clears immune complexes and apoptotic debris; deficiency of an early component impairs clearance and predisposes to pyogenic infection and, characteristically, to immune-complex disease such as lupus-like syndromes, whereas deficiency of a terminal component impairs the membrane attack complex and confers susceptibility to disseminated Neisseria infection (Walport, 2001). Regulatory-protein defects cause unrestrained complement activation. In phagocyte disorders the defect lies in the cells that ingest and kill microbes: in chronic granulomatous disease, mutations in the NADPH oxidase complex abolish the respiratory burst, so phagocytes ingest but cannot kill catalase-positive organisms, leading to recurrent infection and granuloma formation (Holland, 2013). Leukocyte adhesion deficiency instead impairs the integrin-mediated adhesion needed for phagocytes to leave the bloodstream and reach infection (Notarangelo, 2010; Tangye, 2022).

Clinical relevance

The specific infection signatures of these defects, such as recurrent Neisseria infection with terminal complement deficiency or catalase-positive organism infection with chronic granulomatous disease, are central to how innate immunodeficiency is conceptualised. As a reference topic this entry explains those associations; it is educational and not a basis for individual diagnosis or treatment.

Epidemiology

Complement and phagocyte defects together form a minority of inborn errors of immunity but produce some of the most recognisable infection patterns. Chronic granulomatous disease is the best-characterised phagocyte killing defect, and terminal complement deficiency is a recognised cause of recurrent meningococcal disease; both are individually rare (Holland, 2013; Walport, 2001).

Evidence & guidelines

The complement and phagocyte categories are defined within the IUIS classification of inborn errors of immunity (Tangye, 2022). Walport's two-part review remains a standard synthesis of complement biology and deficiency (Walport, 2001), and dedicated reviews summarise chronic granulomatous disease (Holland, 2013) and the broader primary immunodeficiency framework (Notarangelo, 2010).

History

Complement deficiencies were among the earliest recognised innate immune defects, with terminal-component deficiency linked to recurrent neisserial disease. Chronic granulomatous disease, first described in the 1950s as a fatal granulomatous disorder of childhood, was later shown to result from a failure of the phagocyte oxidative burst, providing a paradigm for how an innate killing defect produces a specific infection signature (Walport, 2001; Holland, 2013).

Key figures

• Mark Walport
• Steven Holland
• Luigi Notarangelo
• Stuart Tangye

Seminal works

• tangye-2022
• walport-2001a
• holland-2013
• notarangelo-2010

Frequently asked questions

Why does terminal complement deficiency cause recurrent Neisseria infections?

The terminal complement components (C5-C9) assemble the membrane attack complex, which is particularly important for direct lysis of Neisseria species; without it, the host loses a key defence against these organisms and is prone to recurrent or disseminated neisserial infection.

What goes wrong in chronic granulomatous disease?

Mutations disable the phagocyte NADPH oxidase complex, so neutrophils and macrophages can ingest microbes but cannot generate the reactive oxygen species needed to kill catalase-positive organisms, leading to recurrent infections and inflammatory granulomas.