Late Complement Component Deficiency

Executive Summary

Late complement component deficiency (also known as terminal complement pathway deficiency) refers to inherited deficiencies affecting complement components C5, C6, C7, C8, or C9, which are essential for formation of the membrane attack complex (MAC; C5b-9). These rare, autosomal recessive disorders confer a dramatically increased susceptibility to invasive infections by Neisseria species, particularly Neisseria meningitidis (meningococcemia and meningitis) and Neisseria gonorrhoeae (disseminated gonococcal infection). The disease is characterized by recurrent, potentially fatal invasive bacterial infections despite the ability to mount antibody responses. Timely diagnosis through complement functional assays (CH50/AH50), specific component measurement, and genetic testing enables lifelong preventive strategies including vaccination, antibiotic prophylaxis, and family screening (brodszki2020europeansocietyfor pages 1-2, shears2022clinicaloutcomeand pages 1-2, balduit2023geneticbasesof pages 1-2, mcmurray2024immunodeficiencycomplementdisorders. pages 1-2, brodszki2020europeansocietyfor pages 10-12).

1. Disease Information

Overview

Late complement component deficiency is a group of rare primary immunodeficiencies (PIDs) caused by inherited defects in the terminal complement components C5, C6, C7, C8, or C9. These proteins sequentially assemble the membrane attack complex (MAC or C5b-9), which forms transmembrane pores that lyse Gram-negative bacteria and other target cells (brodszki2020europeansocietyfor pages 1-2, hodeib2020humangeneticsof pages 2-4).

The complement system comprises over 30 proteins and is organized into three activation pathways—classical, lectin, and alternative—that converge at C3 cleavage and then proceed through a common terminal pathway involving C5 through C9. Complement deficiencies collectively account for approximately 5% of all reported primary immunodeficiencies, though they are likely underdiagnosed (brodszki2020europeansocietyfor pages 1-2). The European Society for Immunodeficiencies (ESID) guideline states: "The complement system is a crucial part of the innate immune system, with multiple membrane-bound and soluble components... Complement deficiencies account for ~5% of PIDs" (brodszki2020europeansocietyfor pages 1-2).

Synonyms and Alternative Names

• Terminal complement pathway deficiency
• Terminal complement component deficiency
• MAC deficiency (membrane attack complex deficiency)
• C5-C9 deficiency (component-specific)
• Late-acting complement deficiency

Key Identifiers

OMIM: C6 deficiency has OMIM ID 612446 (li2020novelpathogenicmutations pages 1-2). OMIM identifiers for C5, C7, C8, and C9 deficiencies were not definitively confirmed in the retrieved literature, though these disorders are well-characterized genetically.

Orphanet, ICD-10/11, MeSH, MONDO: Specific coding details were not available in the retrieved sources. The disease would be classified under broader complement deficiency and immunodeficiency categories.

Data Source: Information is derived from aggregated disease-level resources (clinical cohorts, systematic reviews, case series) and molecular genetics studies, not individual patient EHR data (shears2022clinicaloutcomeand pages 1-2, balduit2023geneticbasesof pages 1-2, shears2022clinicaloutcomeand pages 2-5, balduit2023geneticbasesof pages 2-4).

2. Etiology

Disease Causal Factors

Primary Genetic Cause:
Late complement component deficiency results from germline mutations in the genes encoding C5 (chromosome 9q33.2), C6 (5p13), C7 (5p13), C8A/C8B (1p32) and C8G (9q34), or C9 (5p14-p12). These mutations lead to absent or severely reduced serum levels of the affected component, disrupting MAC formation (szymanska2024molecularaspectsof pages 1-2, li2020novelpathogenicmutations pages 1-2, brodszki2020europeansocietyfor pages 5-6).

C8 deficiency involves three subunit genes: C8A and C8B (encoding the alpha and beta chains that form a heterodimer) and C8G (encoding the gamma chain). Deficiency of any subunit can cause functional C8 deficiency (brodszki2020europeansocietyfor pages 5-6).

Mechanistic Basis:
The terminal pathway begins when C5 convertase cleaves C5 into C5a (an inflammatory anaphylatoxin) and C5b. C5b binds C6 and C7, forming a complex that inserts into target membranes. C8 then binds, and multiple C9 molecules polymerize to create the lytic MAC pore. Without functional MAC, serum bactericidal activity against pathogenic Neisseria is lost, despite intact opsonization and antibody responses (hodeib2020humangeneticsof pages 2-4, szymanska2024molecularaspectsof pages 1-2).

Risk Factors

Genetic Risk Factors: - Autosomal recessive inheritance: All terminal complement deficiencies are inherited in an autosomal recessive pattern (brodszki2020europeansocietyfor pages 1-2, brodszki2020europeansocietyfor pages 5-6). - Consanguinity: In a UK multicenter cohort of 40 patients with terminal complement deficiency, 65% had consanguineous parenthood, and 80% had a family history of the condition (shears2022clinicaloutcomeand pages 1-2, shears2022clinicaloutcomeand pages 2-5). - Founder effects: Strong founder effects exist in specific populations: - C7 deficiency: 1:400 in Israeli Moroccan Jews (sullivan2026complementdeficiencies pages 1-45, khalil2023thefirstcase pages 1-2) - C6 deficiency: 1:1400 in African Americans (sullivan2026complementdeficiencies pages 1-45) - C9 deficiency: 1:1000 in Japanese populations (sullivan2026complementdeficiencies pages 1-45, li2020novelpathogenicmutations pages 1-2)

Environmental/Exposure Risk Factors: - Geographic residence in meningitis-endemic areas (e.g., sub-Saharan "meningitis belt") increases exposure risk (hodeib2020humangeneticsof pages 2-4). - Occupational or social exposure to Neisseria (e.g., nursery workers, crowded living conditions) increases infection risk once deficiency is present (brodszki2020europeansocietyfor pages 10-12).

Protective Factors

No intrinsic protective genetic variants within complement genes have been identified for terminal pathway deficiency. However, preventive interventions (vaccination, antibiotic prophylaxis, family screening) markedly reduce disease burden (see Section 12).

Gene-Environment Interactions

Terminal complement deficiency confers susceptibility, but invasive disease requires environmental exposure to pathogenic Neisseria. Vaccination and hygiene measures reduce exposure, demonstrating gene-environment interaction in disease expression (brodszki2020europeansocietyfor pages 10-12).

3. Phenotypes

Table: This table summarizes the main clinical manifestations and laboratory phenotypes of late/terminal complement component deficiency (C5-C9), with HPO suggestions, frequencies, onset patterns, and mechanistic notes. It is useful for phenotype curation and clinical knowledge-base population.

Key Clinical Phenotypes

Recurrent Invasive Meningococcal Disease is the hallmark phenotype. Patients have a 7,000- to 10,000-fold increased risk of invasive meningococcal disease compared to the general population (shears2022clinicaloutcomeand pages 1-2). In a large UK multicenter cohort, 48% had a history of meningococcal or other deep-seated bacterial infections, with median age at first infection of 9 years (range 1-25 years) (shears2022clinicaloutcomeand pages 1-2, shears2022clinicaloutcomeand pages 2-5).

Suggested HPO Terms: - HP:0005381 Recurrent bacterial infections - HP:0002718 Recurrent invasive bacterial infection - HP:0002013 Meningitis - HP:0100806 Sepsis - HP:0012378 Abnormal complement system physiology - HP:0033704 Reduced complement activity

Laboratory Phenotype:
Absent or markedly reduced CH50 (classical pathway) and AH50 (alternative pathway) hemolytic activity is diagnostic. For example, a C7-deficient patient showed classical pathway activity 0.6%, lectin 0.2%, and alternative 0.1% of normal (balduit2023geneticbasesof pages 1-2, balduit2023geneticbasesof pages 2-4). Western blot or ELISA typically confirms absence of the specific component protein (balduit2023geneticbasesof pages 1-2).

Age of Onset:
Most infections occur in childhood or adolescence, though adult-onset cases are reported. The median age at first infection was 9 years in the UK cohort (shears2022clinicaloutcomeand pages 2-5).

Severity and Progression:
Severity is variable. Acute episodes can be fulminant, requiring intensive care (22% in the UK cohort) (shears2022clinicaloutcomeand pages 2-5). However, many patients remain asymptomatic until first exposure; 52% in the UK cohort were diagnosed only through family screening (shears2022clinicaloutcomeand pages 2-5).

Quality of Life Impact:
Recurrent life-threatening infections, repeated hospitalizations, and fear of meningitis impose substantial psychosocial burden. Prophylactic regimens (vaccinations, daily antibiotics, emergency planning) require lifelong adherence, affecting daily routines and quality of life (khalil2023thefirstcase pages 1-2, brodszki2020europeansocietyfor pages 10-12).

4. Genetic/Molecular Information

Table: This table summarizes the principal causal genes underlying late/terminal complement component deficiency (C5-C9), including chromosomal location, inheritance, recurrent variant classes, and notable population founder effects. It is useful for genetics-focused disease curation and for prioritizing molecular testing in suspected terminal pathway deficiency.

Causal Genes

• C5 gene (9q33.2): Encodes complement C5; 41 exons encoding a pre-protein processed into alpha and beta chains (szymanska2024molecularaspectsof pages 1-2, lizamamunoz2025descriptionandphenotype pages 1-2).
• C6 gene (5p13): 18 exons; deficiency classified as complete (C6Q0) or subtotal (C6SD) (li2020novelpathogenicmutations pages 1-2).
• C7 gene (5p13): Mutations include missense, nonsense, splice-site, and regulatory variants (balduit2023geneticbasesof pages 1-2, balduit2023geneticbasesof pages 2-4).
• C8A, C8B, C8G genes: C8A and C8B on 1p32; C8G on 9q34 (brodszki2020europeansocietyfor pages 5-6).
• C9 gene (5p14-p12): Encodes the polymerizing component of MAC (brodszki2020europeansocietyfor pages 5-6).

Pathogenic Variants

A systematic review of C7 deficiency identified diverse mutation types globally (balduit2023geneticbasesof pages 1-2, balduit2023geneticbasesof pages 2-4). Representative pathogenic variants include:

C5: - p.Ile238Thr and p.Gly650Val (compound heterozygous in a family; p.Gly650Val is novel) (lizamamunoz2025descriptionandphenotype pages 1-2) - p.Y352C, p.A252T, p.Q19X, p.R1482X, p.K372R (hodeib2020humangeneticsof pages 2-4)

C6: - p.Arg596Ter and p.Arg606Ter (compound heterozygous nonsense mutations in the first Chinese C6Q0 pedigree) (li2020novelpathogenicmutations pages 1-2)

C7: - p.Gly379Arg (homozygous missense; founder mutation in Moroccan-origin Israeli Jews and reported in Qatar) (khalil2023thefirstcase pages 1-2) - Novel 3'UTR deletion (c.99_101delTCT) causing mRNA instability, leading to functional hemizygosity when combined with coding variant (balduit2023geneticbasesof pages 1-2, balduit2023geneticbasesof pages 2-4)

C8: - p.Thr542Ile in C8B (brodszki2020europeansocietyfor pages 5-6)

Variant Classification:
Pathogenic variants are classified using ACMG/AMP guidelines. Loss-of-function variants (nonsense, frameshift) and missense variants disrupting protein structure/function are typically pathogenic or likely pathogenic (li2020novelpathogenicmutations pages 1-2, lizamamunoz2025descriptionandphenotype pages 1-2).

Allele Frequency:
Terminal complement deficiencies are rare in the general population (prevalence ~0.03%) but enriched in founder populations (brodszki2020europeansocietyfor pages 1-2, sullivan2026complementdeficiencies pages 1-45).

Somatic vs Germline:
All reported cases involve germline mutations; somatic complement deficiency is not a recognized entity for terminal components (balduit2023geneticbasesof pages 1-2, li2020novelpathogenicmutations pages 1-2).

Functional Consequences:
Mutations cause loss of function (reduced or absent protein secretion, misfolding, mRNA instability). The result is absent or nonfunctional MAC assembly (balduit2023geneticbasesof pages 1-2, szymanska2024molecularaspectsof pages 1-2, li2020novelpathogenicmutations pages 1-2).

Modifier Genes

Secondary terminal pathway deficiency can result from excessive C5 consumption due to mutations in complement regulators (Factor H, Factor I). In the UK cohort, 10/16 patients with low antigenic C5 had underlying CFH or CFI mutations rather than primary C5 deficiency (shears2022clinicaloutcomeand pages 1-2, shears2022clinicaloutcomeand pages 2-5).

5. Environmental Information

Environmental Factors:
Exposure to Neisseria meningitidis or Neisseria gonorrhoeae is required for clinical disease expression. Environmental triggers include crowded living conditions, smoking, and prior viral respiratory infections that disrupt mucosal barriers (hodeib2020humangeneticsof pages 2-4).

Lifestyle Factors:
No specific lifestyle factors cause terminal complement deficiency, but behaviors affecting Neisseria exposure (sexual activity for gonorrhea, close-contact settings for meningococcus) modulate disease expression.

Infectious Agents:
- Neisseria meningitidis (serogroups B, C, Y, W, A, E): Gram-negative diplococcus; primary pathogen (khalil2023thefirstcase pages 1-2, shears2022clinicaloutcomeand pages 2-5, hodeib2020humangeneticsof pages 2-4) - Neisseria gonorrhoeae: Causes disseminated gonococcal infection in terminal deficiency (hodeib2020humangeneticsof pages 1-2)

6. Mechanism / Pathophysiology

Molecular Pathways

Terminal Complement Cascade (KEGG, Reactome pathways):
The terminal pathway is initiated by C5 convertase enzymes (C4b2a3b from classical/lectin pathways; C3bBb3b from alternative pathway). C5 convertase cleaves C5 into C5a and C5b. C5b binds sequentially to C6, C7, C8, and multiple C9 molecules, forming the MAC (hodeib2020humangeneticsof pages 2-4, szymanska2024molecularaspectsof pages 1-2).

Suggested Pathway Terms: - Reactome: Complement cascade - KEGG: hsa04610 Complement and coagulation cascades - GO:0006958 Complement activation, classical pathway - GO:0006957 Complement activation, alternative pathway - GO:0006956 Complement activation

Cellular Processes

Impaired Bacterial Lysis:
MAC creates transmembrane pores in bacterial membranes, causing osmotic lysis. Without functional MAC, Neisseria and other Gram-negative bacteria evade complement-mediated killing despite opsonization (hodeib2020humangeneticsof pages 2-4).

Inflammatory Signaling:
C5a (anaphylatoxin) is still generated if C5 is cleaved, contributing to inflammation. However, in complete C5 deficiency, neither C5a nor MAC is formed (szymanska2024molecularaspectsof pages 1-2).

Suggested GO Terms: - GO:0051603 Proteolysis involved in cellular protein catabolic process (MAC-mediated lysis) - GO:0006952 Defense response - GO:0045087 Innate immune response

Protein Dysfunction

Mutations cause: - Absent protein secretion (nonsense, frameshift) - Misfolding and degradation (missense variants disrupting hydrophobic cores or structural domains) - mRNA instability (UTR mutations) (balduit2023geneticbasesof pages 1-2, lizamamunoz2025descriptionandphenotype pages 1-2)

For example, the p.Ile238Thr C5 variant introduces a polar residue into a hydrophobic core, disrupting folding (lizamamunoz2025descriptionandphenotype pages 1-2).

Biochemical Abnormalities

Absent MAC Formation:
The core defect is failure to form C5b-9 complexes. Hemolytic assays (CH50, AH50) detect this functional deficiency (balduit2023geneticbasesof pages 1-2, balduit2023geneticbasesof pages 2-4).

Impaired Serum Bactericidal Activity (SBA):
Serum from deficient individuals cannot kill Neisseria in vitro, despite normal antibody levels (hodeib2020humangeneticsof pages 2-4, szymanska2024molecularaspectsof pages 1-2).

Immune System Involvement

Immunodeficiency Phenotype:
Patients have intact adaptive immunity (antibody production, T-cell responses) but lack complement-mediated bacterial killing. This is a selective defect in innate immunity (brodszki2020europeansocietyfor pages 1-2, hodeib2020humangeneticsof pages 2-4).

Suggested Cell Types (Cell Ontology): - CL:0000542 Lymphocyte (adaptive immunity intact) - CL:0000623 Natural killer cell - CL:0000235 Macrophage (phagocytosis preserved but complement-enhanced killing impaired)

Causal Chain

1. Germline mutation in C5, C6, C7, C8, or C9 gene
2. Absent or nonfunctional protein secretion/stability
3. Failure to assemble MAC (C5b-9)
4. Loss of serum bactericidal activity against Neisseria
5. Invasive meningococcal or gonococcal infection upon exposure
6. Clinical manifestations: sepsis, meningitis, arthritis, chronic meningococcemia

7. Anatomical Structures Affected

Organ Level

Primary Organs: - Central nervous system: Meningitis (leptomeninges, brain, spinal cord) (khalil2023thefirstcase pages 1-2, shears2022clinicaloutcomeand pages 2-5) - Blood/vascular system: Septicemia, disseminated intravascular coagulation (shears2022clinicaloutcomeand pages 1-2, shears2022clinicaloutcomeand pages 2-5)

Secondary Organs: - Joints: Septic arthritis, meningococcal arthritis (shears2022clinicaloutcomeand pages 2-5) - Skin: Petechial rash, purpura fulminans (khalil2023thefirstcase pages 1-2, balduit2023geneticbasesof pages 2-4) - Reproductive system: Disseminated gonococcal infection (hodeib2020humangeneticsof pages 1-2)

Suggested UBERON Terms: - UBERON:0002423 leptomeninges - UBERON:0000955 brain - UBERON:0000178 blood - UBERON:0000982 skeletal joint - UBERON:0002097 skin of body

Tissue and Cell Level

Cell Populations Targeted by Pathogen: - Endothelial cells (sites of Neisseria invasion) (hodeib2020humangeneticsof pages 2-4) - Cells of the meninges and blood-brain barrier

Suggested Cell Ontology Terms: - CL:0000115 Endothelial cell - CL:0000125 Glial cell (in CNS infection)

Subcellular Level

MAC normally inserts into bacterial outer membranes. In deficiency, this process fails at the plasma membrane/cell surface level.

Suggested GO Cellular Component Terms: - GO:0005886 Plasma membrane - GO:0016020 Membrane

8. Temporal Development

Onset

Typical Age of Onset:
Usually pediatric to young adult. Median age at first infection was 9 years (range 1-25) in the UK cohort (shears2022clinicaloutcomeand pages 2-5). However, asymptomatic individuals can present at any age upon first Neisseria exposure.

Onset Pattern:
Acute (invasive infections develop rapidly) (khalil2023thefirstcase pages 1-2, shears2022clinicaloutcomeand pages 1-2).

Progression

Disease Course:
Episodic and recurrent. Without diagnosis and prevention, patients may experience multiple meningococcal infections over years (e.g., a Qatar case had 3 episodes at ages 7, 18, and 20) (khalil2023thefirstcase pages 1-2).

Disease Duration:
Chronic lifelong susceptibility (brodszki2020europeansocietyfor pages 10-12).

Progression Rate:
Acute invasive infections; intervals between episodes variable, depending on exposure and prevention measures (shears2022clinicaloutcomeand pages 2-5).

Patterns

Remission:
Infection-free periods occur spontaneously but reflect absence of exposure rather than true remission. Effective prevention (vaccination, prophylaxis) markedly extends infection-free survival (khalil2023thefirstcase pages 1-2, brodszki2020europeansocietyfor pages 10-12).

Critical Periods:
Childhood/adolescence (high meningococcal carriage rates in populations). Post-diagnosis preventive management is a critical window for lifelong risk reduction (brodszki2020europeansocietyfor pages 10-12).

9. Inheritance and Population

Epidemiology

Prevalence:
Combined prevalence of complement deficiencies (all types) is approximately 0.03% in the general population. Terminal complement deficiencies are rarer than early pathway deficiencies (brodszki2020europeansocietyfor pages 1-2). Founder populations show higher prevalence (see Founder Effects below).

Incidence:
Specific incidence rates not well-defined in recent literature. Underdiagnosis is likely (brodszki2020europeansocietyfor pages 1-2).

Inheritance Pattern

Autosomal Recessive:
All terminal complement deficiencies (C5-C9) are autosomal recessive. Heterozygotes are typically asymptomatic carriers (brodszki2020europeansocietyfor pages 1-2, li2020novelpathogenicmutations pages 1-2, brodszki2020europeansocietyfor pages 5-6).

Penetrance:
Incomplete penetrance for infection: not all individuals with deficiency develop invasive disease, depending on Neisseria exposure. However, lifetime infection risk is very high (shears2022clinicaloutcomeand pages 1-2, shears2022clinicaloutcomeand pages 2-5).

Expressivity:
Variable; some patients have recurrent infections, others remain asymptomatic (shears2022clinicaloutcomeand pages 2-5).

Genetic Anticipation:
Not observed (brodszki2020europeansocietyfor pages 1-2).

Germline Mosaicism:
Not specifically reported for terminal complement deficiencies.

Founder Effects: - C7 deficiency: 1:400 in Israeli Moroccan Jews (sullivan2026complementdeficiencies pages 1-45, khalil2023thefirstcase pages 1-2) - C6 deficiency: 1:1400 in African Americans; high prevalence in Western Cape, South Africa (sullivan2026complementdeficiencies pages 1-45, li2020novelpathogenicmutations pages 1-2) - C9 deficiency: 1:1000 in Japanese (sullivan2026complementdeficiencies pages 1-45, li2020novelpathogenicmutations pages 1-2)

Consanguinity:
Strongly associated. In the UK cohort, 65% of patients had consanguineous parents (shears2022clinicaloutcomeand pages 1-2, shears2022clinicaloutcomeand pages 2-5).

Carrier Frequency:
Not precisely quantified in general populations but elevated in founder populations (sullivan2026complementdeficiencies pages 1-45).

Population Demographics

Affected Populations:
Enriched in populations with founder mutations (Moroccan Jews, African ancestry, Japanese). The UK cohort was 68% Asian (likely Pakistani/Bangladeshi given UK demographics and consanguinity patterns) and 32% white European (shears2022clinicaloutcomeand pages 1-2, shears2022clinicaloutcomeand pages 2-5).

Geographic Distribution:
Worldwide, but recognized clusters in: - North Africa (C7 in Moroccans) - Sub-Saharan Africa (C6) - East Asia (C9 in Japan) - UK (Asian consanguineous communities)

Sex Ratio:
In the UK cohort, 55% were male, 45% female, consistent with autosomal recessive inheritance (no sex bias expected) (shears2022clinicaloutcomeand pages 1-2, shears2022clinicaloutcomeand pages 2-5).

Age Distribution:
Diagnosed at median age 14 years (range 1-45) in the UK cohort; 52% were asymptomatic at diagnosis (family-screened) (shears2022clinicaloutcomeand pages 2-5).

10. Diagnostics

Clinical Tests

Hemolytic Complement Assays: - CH50 (classical pathway): Absent or very low (<1% activity typical in terminal deficiency) (balduit2023geneticbasesof pages 1-2, balduit2023geneticbasesof pages 2-4) - AH50 (alternative pathway): Absent or very low (balduit2023geneticbasesof pages 1-2) - Suggested LOINC codes: CH50, AH50 (specific codes require consultation with LOINC database)

Specific Component Quantification: - ELISA or radial immunodiffusion (Ouchterlony) for C5, C6, C7, C8, C9 protein levels (shears2022clinicaloutcomeand pages 1-2, balduit2023geneticbasesof pages 1-2, li2020novelpathogenicmutations pages 1-2) - Western blot confirms absence of specific component (balduit2023geneticbasesof pages 1-2, balduit2023geneticbasesof pages 2-4)

Biomarkers: - Absent C5, C6, C7, C8, or C9 antigen - Absent CH50/AH50 activity - Normal C3, C4 (distinguishes from early pathway or regulatory defects) (sullivan2026complementdeficiencies pages 1-45, balduit2023geneticbasesof pages 1-2)

Genetic Testing

Recommended Approach:
Genetic testing is increasingly standard after functional diagnosis. Next-generation sequencing (targeted gene panels, whole exome sequencing) identifies pathogenic variants (balduit2023geneticbasesof pages 1-2, li2020novelpathogenicmutations pages 1-2).

Single Gene Testing:
Sanger sequencing of candidate gene (C5, C6, C7, C8A/B/G, C9) based on which component is absent (balduit2023geneticbasesof pages 1-2, balduit2023geneticbasesof pages 2-4, li2020novelpathogenicmutations pages 1-2).

Whole Exome/Genome Sequencing:
Useful when functional testing suggests terminal deficiency but specific component not determined, or for complex cases (li2020novelpathogenicmutations pages 1-2).

Important Note:
Patients with low antigenic C5 may have secondary deficiency from CFH or CFI mutations (excessive consumption). Genetic testing clarifies primary vs. secondary deficiency (shears2022clinicaloutcomeand pages 1-2, shears2022clinicaloutcomeand pages 2-5).

Clinical Criteria

Diagnostic Criteria: 1. Recurrent invasive Neisseria infections (especially atypical serotypes) 2. Absent or very low CH50 and AH50 3. Normal C3, C4 (rules out early pathway defects) 4. Absent specific terminal component by ELISA/Western blot 5. Pathogenic variant(s) in corresponding gene

Differential Diagnosis: - Properdin or Factor D deficiency: Also predispose to Neisseria but affect alternative pathway specifically; AH50 absent, CH50 may be normal or reduced (sullivan2026complementdeficiencies pages 1-45) - Secondary complement consumption: Low C5 from CFH/CFI defects (atypical HUS phenotype may coexist) (shears2022clinicaloutcomeand pages 1-2, shears2022clinicaloutcomeand pages 2-5) - Acquired complement deficiency: Autoantibodies, liver disease (brodszki2020europeansocietyfor pages 1-2)

Screening

Family Screening:
Strongly recommended for siblings of index cases. In the UK cohort, 80% had a family history, and 52% were asymptomatic (shears2022clinicaloutcomeand pages 1-2, shears2022clinicaloutcomeand pages 2-5, brodszki2020europeansocietyfor pages 10-12).

Newborn Screening:
Not routinely performed for complement deficiency.

Cascade Screening:
After index case diagnosis, test siblings and consider extended family if consanguinity present (brodszki2020europeansocietyfor pages 10-12).

11. Outcome/Prognosis

Survival and Mortality

Mortality:
In the UK cohort of 40 patients, 2 (5%) died. One death was directly attributable to fulminant pneumococcal meningitis in a 4-month-old with Factor I deficiency; the other was from COVID-19 in an adult with C8 deficiency (shears2022clinicaloutcomeand pages 1-2, shears2022clinicaloutcomeand pages 2-5).

Life Expectancy:
With modern preventive management (vaccination, prophylaxis), life expectancy can approach normal. Without recognition, recurrent meningococcal infections carry high mortality (khalil2023thefirstcase pages 1-2, brodszki2020europeansocietyfor pages 10-12).

Morbidity and Function

Morbidity:
Recurrent ICU admissions (22% in UK cohort), potential for neurologic sequelae from meningitis, joint damage from septic arthritis (shears2022clinicaloutcomeand pages 1-2, shears2022clinicaloutcomeand pages 2-5).

Quality of Life:
Burden from lifelong medication adherence, fear of infection, repeated vaccinations, emergency planning. A well-managed C7-deficient patient remained infection-free over 10 years, illustrating good quality of life achievable with prevention (khalil2023thefirstcase pages 1-2).

Disease Course

Complications:
- Purpura fulminans, DIC (shears2022clinicaloutcomeand pages 1-2) - Neurologic sequelae from meningitis (deafness, cognitive impairment) - Septic arthritis - Death from fulminant sepsis (shears2022clinicaloutcomeand pages 2-5)

Recovery Potential:
Most patients recover fully from individual infection episodes with prompt treatment, though neurologic damage may be permanent (khalil2023thefirstcase pages 1-2, shears2022clinicaloutcomeand pages 1-2).

Prognostic Factors

Poor Prognostic Factors: - Delay to diagnosis - Fulminant septicemia (vs. meningitis alone) - Lack of vaccination/prophylaxis - Young age (infants more vulnerable) (shears2022clinicaloutcomeand pages 1-2, shears2022clinicaloutcomeand pages 2-5)

Good Prognostic Factors: - Early diagnosis (especially via family screening before first infection) - Comprehensive vaccination - Adherence to prophylaxis - Emergency planning (khalil2023thefirstcase pages 1-2, brodszki2020europeansocietyfor pages 10-12)

12. Treatment

Table: This table summarizes the main prevention and management interventions for late complement component deficiency, with emphasis on vaccination, antibiotic strategies, family screening, and emergency planning. It is useful for translating the literature into actionable clinical knowledge-base entries with ontology-ready intervention labels.

Pharmacotherapy

Prophylactic Antibiotics:
Penicillin or amoxicillin is commonly used. Macrolides (azithromycin) are alternatives. In the UK cohort, 70% were on prophylaxis, though 22% admitted noncompliance (shears2022clinicaloutcomeand pages 2-5). A C7-deficient patient on azithromycin plus vaccination remained infection-free for 10 years (khalil2023thefirstcase pages 1-2).

Suggested MAXO Terms: - MAXO:0000756 Prophylactic antibiotic therapy - MAXO:0001039 Antibacterial therapy

Acute Treatment:
Standard management of meningococcal sepsis (IV ceftriaxone, supportive care, ICU as needed) (khalil2023thefirstcase pages 1-2, shears2022clinicaloutcomeand pages 2-5).

Advanced Therapeutics

Fresh Frozen Plasma:
Rarely used for acute severe infection (2 patients in UK cohort received FFP for fulminant meningococcemia). Not practical for long-term prevention (shears2022clinicaloutcomeand pages 2-5, sullivan2026complementdeficiencies pages 1-45).

Gene Therapy / Cell Therapy:
Not currently available for terminal complement deficiency. Liver transplantation could theoretically cure (liver produces complement), but not justified for this indication (sullivan2026complementdeficiencies pages 1-45).

Vaccination

Meningococcal Vaccines:
- MenACWY conjugate: Strongly recommended, boosters every 3-5 years (brodszki2020europeansocietyfor pages 10-12) - MenB: Recommended (43% of UK cohort infections were serogroup B) (shears2022clinicaloutcomeand pages 2-5, brodszki2020europeansocietyfor pages 10-12) - In UK cohort, 98% received booster vaccinations post-diagnosis (shears2022clinicaloutcomeand pages 2-5)

Pneumococcal Vaccine:
Conjugate pneumococcal vaccine recommended (15% received extra dose in UK cohort) (shears2022clinicaloutcomeand pages 2-5, brodszki2020europeansocietyfor pages 10-12).

Haemophilus influenzae type b:
Routine vaccination recommended (brodszki2020europeansocietyfor pages 10-12).

Efficacy:
Vaccination may halve meningococcal disease risk but does not eliminate it. Breakthrough infections occur, so vaccination must be combined with prophylaxis and emergency planning (sullivan2026complementdeficiencies pages 1-45, brodszki2020europeansocietyfor pages 10-12).

Suggested MAXO Terms: - MAXO:0000758 Meningococcal vaccination - MAXO:0000757 Pneumococcal vaccination - MAXO:0001309 Haemophilus influenzae vaccination

Supportive Care

Emergency Planning:
All patients should have: - MedicAlert or similar identification - Emergency antibiotic supply - Written action plan for fever/suspected infection - Annual review with immunology specialist (brodszki2020europeansocietyfor pages 10-12)

Suggested MAXO Terms: - MAXO:0000011 Patient education - MAXO:0000004 Emergency care planning

Treatment Outcomes

Response Rates:
With comprehensive prevention (vaccination + prophylaxis + family screening), the 10-year follow-up of a C7-deficient patient showed no recurrent infections (khalil2023thefirstcase pages 1-2). Mortality is low with modern management (<5% in UK cohort over median follow-up period) (shears2022clinicaloutcomeand pages 2-5).

Adverse Events:
Antibiotic resistance from chronic prophylaxis is a concern. Noncompliance with prophylaxis was 22% in the UK cohort (shears2022clinicaloutcomeand pages 2-5, brodszki2020europeansocietyfor pages 10-12).

13. Prevention

Primary Prevention

Vaccination:
MenACWY, MenB, pneumococcal, Hib vaccines reduce infection risk (see Section 12) (brodszki2020europeansocietyfor pages 10-12).

Antibiotic Prophylaxis:
Penicillin or macrolides for high-risk individuals (recurrent infections, endemic areas) (brodszki2020europeansocietyfor pages 10-12).

Secondary Prevention

Family Screening:
Cascade testing of siblings identifies asymptomatic at-risk individuals before first infection. In UK cohort, 52% were asymptomatic at diagnosis (shears2022clinicaloutcomeand pages 1-2, shears2022clinicaloutcomeand pages 2-5, brodszki2020europeansocietyfor pages 10-12).

Tertiary Prevention

Emergency Planning:
Rapid recognition and treatment of infections prevents complications (brodszki2020europeansocietyfor pages 10-12).

Contact Vaccination:
Household members and close contacts should also receive meningococcal vaccines (brodszki2020europeansocietyfor pages 10-12).

Genetic Counseling

Autosomal recessive inheritance counseling for families. Carrier testing for siblings, prenatal/preimplantation diagnosis possible but rarely requested given good outcomes with prevention (brodszki2020europeansocietyfor pages 10-12).

Suggested MAXO Terms: - MAXO:0000503 Genetic counseling - MAXO:0000009 Family member screening

14. Other Species / Natural Disease

Naturally Occurring Disease in Animals

Terminal complement deficiencies have been studied in animal models but are not well-documented as naturally occurring diseases in veterinary medicine. Mice and other mammals have orthologous C5-C9 genes.

Suggested NCBI Taxon IDs: - Mus musculus (house mouse): 10090 - Rattus norvegicus (Norway rat): 10116

Comparative Biology

Complement is highly conserved across vertebrates. MAC-mediated bacterial killing is a fundamental defense mechanism in mammals, birds, and fish (brodszki2020europeansocietyfor pages 1-2).

15. Model Organisms

Mammalian Models

Mouse Models: - C5-deficient mice (spontaneous mutants and knockouts) have impaired bacterial clearance and increased susceptibility to Neisseria infection in experimental models (inferred from complement system biology literature). - C6-deficient mice and C9-deficient mice have been generated and studied for complement biology.

Applications:
Mouse models enable study of MAC function in bacterial clearance, inflammation, and immune regulation. However, mice do not naturally harbor Neisseria meningitidis, limiting translational modeling (general complement biology knowledge).

Suggested Resources: - MGI (Mouse Genome Informatics): For C5, C6, C7, C8a, C8b, C8g, C9 knockout/mutant strains - IMPC (International Mouse Phenotyping Consortium): For phenotyping data

In Vitro Models

Cell Lines:
Human serum bactericidal assays using patient vs. normal serum are standard for assessing functional deficiency (hodeib2020humangeneticsof pages 2-4).

Organoids / iPSCs:
Not specifically reported for terminal complement deficiency modeling.

Conclusions

Late complement component deficiency (C5-C9 deficiency) is a rare, autosomal recessive primary immunodeficiency characterized by absent or nonfunctional membrane attack complex (MAC), leading to markedly increased susceptibility to invasive Neisseria infections. The disease presents with recurrent meningococcal meningitis and/or septicemia, often beginning in childhood or adolescence. Diagnosis relies on functional complement assays (absent CH50/AH50) and genetic confirmation of pathogenic variants in C5, C6, C7, C8, or C9 genes. Founder effects exist in Israeli Moroccan Jews (C7), African Americans (C6), and Japanese populations (C9).

Management requires lifelong preventive strategies including meningococcal (MenACWY, MenB) and pneumococcal vaccination, antibiotic prophylaxis, family screening, and emergency planning. With comprehensive prevention, patients can achieve near-normal life expectancy and quality of life, as demonstrated by a C7-deficient patient remaining infection-free over 10 years with vaccination and azithromycin prophylaxis. Breakthrough infections remain possible, underscoring the need for vigilance and adherence to prevention protocols. This report synthesizes recent (2020-2025) literature, emphasizing genetic bases, clinical phenotypes, diagnostic approaches, and evidence-based management strategies for knowledge base curation (brodszki2020europeansocietyfor pages 1-2, khalil2023thefirstcase pages 1-2, shears2022clinicaloutcomeand pages 1-2, balduit2023geneticbasesof pages 1-2, shears2022clinicaloutcomeand pages 2-5, mcmurray2024immunodeficiencycomplementdisorders. pages 1-2, brodszki2020europeansocietyfor pages 10-12).

Key References: - Brodszki et al. 2020: ESID/ERN RITA Complement Guideline (brodszki2020europeansocietyfor pages 1-2, brodszki2020europeansocietyfor pages 10-12) - Shears et al. 2022: UK multicenter cohort of terminal complement deficiency (shears2022clinicaloutcomeand pages 1-2, shears2022clinicaloutcomeand pages 2-5) - Balduit et al. 2023: C7 deficiency systematic review and novel mutation (balduit2023geneticbasesof pages 1-2, balduit2023geneticbasesof pages 2-4) - Khalil et al. 2023: C7 deficiency case with 10-year follow-up (khalil2023thefirstcase pages 1-2) - Hodeib et al. 2020: Human genetics of meningococcal infections (hodeib2020humangeneticsof pages 2-4) - Li et al. 2020: First Chinese C6 deficiency pedigree (li2020novelpathogenicmutations pages 1-2) - Szymańska 2024: C5 deficiency molecular review (szymanska2024molecularaspectsof pages 1-2) - Lizama-Muñoz et al. 2025: Novel C5 variant family report (lizamamunoz2025descriptionandphenotype pages 1-2) - McMurray et al. 2024: Complement disorders overview (mcmurray2024immunodeficiencycomplementdisorders. pages 1-2) - Sullivan 2026: Complement deficiencies teaching resource (sullivan2026complementdeficiencies pages 1-45)

References

1. (brodszki2020europeansocietyfor pages 1-2): Nicholas Brodszki, Ashley Frazer-Abel, Anete S. Grumach, Michael Kirschfink, Jiri Litzman, Elena Perez, Mikko R. J. Seppänen, Kathleen E. Sullivan, and Stephen Jolles. European society for immunodeficiencies (esid) and european reference network on rare primary immunodeficiency, autoinflammatory and autoimmune diseases (ern rita) complement guideline: deficiencies, diagnosis, and management. Journal of Clinical Immunology, 40:576-591, Feb 2020. URL: https://doi.org/10.1007/s10875-020-00754-1, doi:10.1007/s10875-020-00754-1. This article has 117 citations and is from a domain leading peer-reviewed journal.

2. (shears2022clinicaloutcomeand pages 1-2): Annalie Shears, Cathal Steele, Jamie Craig, Stephen Jolles, Sinisa Savic, Rosie Hague, Tanya Coulter, Richard Herriot, and Peter D. Arkwright. Clinical outcome and underlying genetic cause of functional terminal complement pathway deficiencies in a multicenter uk cohort. Journal of Clinical Immunology, 42:665-671, Jan 2022. URL: https://doi.org/10.1007/s10875-022-01213-9, doi:10.1007/s10875-022-01213-9. This article has 5 citations and is from a domain leading peer-reviewed journal.

3. (balduit2023geneticbasesof pages 1-2): Andrea Balduit, Anna Monica Bianco, Alessandro Mangogna, Anna Maria Zicari, Lucia Leonardi, Bianca Laura Cinicola, Martina Capponi, Alberto Tommasini, Chiara Agostinis, Adamo Pio d’Adamo, and Roberta Bulla. Genetic bases of c7 deficiency: systematic review and report of a novel deletion determining functional hemizygosity. Frontiers in Immunology, May 2023. URL: https://doi.org/10.3389/fimmu.2023.1192690, doi:10.3389/fimmu.2023.1192690. This article has 8 citations and is from a peer-reviewed journal.

4. (mcmurray2024immunodeficiencycomplementdisorders. pages 1-2): Jeremy C. McMurray, Brandon J. Schornack, Andrew L. Weskamp, Katherine J. Park, Joshua D. Pollock, W. Grant Day, Aaron T. Brockshus, Douglas E. Beakes, David J. Schwartz, Cecilia P. Mikita, and Luke M. Pittman. Immunodeficiency: complement disorders. Allergy and asthma proceedings, 45 5:305-309, Sep 2024. URL: https://doi.org/10.2500/aap.2024.45.240050, doi:10.2500/aap.2024.45.240050. This article has 23 citations and is from a peer-reviewed journal.

5. (brodszki2020europeansocietyfor pages 10-12): Nicholas Brodszki, Ashley Frazer-Abel, Anete S. Grumach, Michael Kirschfink, Jiri Litzman, Elena Perez, Mikko R. J. Seppänen, Kathleen E. Sullivan, and Stephen Jolles. European society for immunodeficiencies (esid) and european reference network on rare primary immunodeficiency, autoinflammatory and autoimmune diseases (ern rita) complement guideline: deficiencies, diagnosis, and management. Journal of Clinical Immunology, 40:576-591, Feb 2020. URL: https://doi.org/10.1007/s10875-020-00754-1, doi:10.1007/s10875-020-00754-1. This article has 117 citations and is from a domain leading peer-reviewed journal.

6. (hodeib2020humangeneticsof pages 2-4): Stephanie Hodeib, Jethro A. Herberg, Michael Levin, and Vanessa Sancho-Shimizu. Human genetics of meningococcal infections. Human Genetics, 139:961-980, Feb 2020. URL: https://doi.org/10.1007/s00439-020-02128-4, doi:10.1007/s00439-020-02128-4. This article has 40 citations and is from a peer-reviewed journal.

7. (li2020novelpathogenicmutations pages 1-2): Philip H Li, William WY Wong, Evelyn NY Leung, Chak‐sing Lau, and Elaine Au. Novel pathogenic mutations identified in the first chinese pedigree of complete c6 deficiency. Clinical & Translational Immunology, Jul 2020. URL: https://doi.org/10.1002/cti2.1148, doi:10.1002/cti2.1148. This article has 8 citations and is from a peer-reviewed journal.

8. (shears2022clinicaloutcomeand pages 2-5): Annalie Shears, Cathal Steele, Jamie Craig, Stephen Jolles, Sinisa Savic, Rosie Hague, Tanya Coulter, Richard Herriot, and Peter D. Arkwright. Clinical outcome and underlying genetic cause of functional terminal complement pathway deficiencies in a multicenter uk cohort. Journal of Clinical Immunology, 42:665-671, Jan 2022. URL: https://doi.org/10.1007/s10875-022-01213-9, doi:10.1007/s10875-022-01213-9. This article has 5 citations and is from a domain leading peer-reviewed journal.

9. (balduit2023geneticbasesof pages 2-4): Andrea Balduit, Anna Monica Bianco, Alessandro Mangogna, Anna Maria Zicari, Lucia Leonardi, Bianca Laura Cinicola, Martina Capponi, Alberto Tommasini, Chiara Agostinis, Adamo Pio d’Adamo, and Roberta Bulla. Genetic bases of c7 deficiency: systematic review and report of a novel deletion determining functional hemizygosity. Frontiers in Immunology, May 2023. URL: https://doi.org/10.3389/fimmu.2023.1192690, doi:10.3389/fimmu.2023.1192690. This article has 8 citations and is from a peer-reviewed journal.

10. (szymanska2024molecularaspectsof pages 1-2): Hanna Szymańska. Molecular aspects of hereditary complement component c5 deficiency in humans. Polish Annals of Medicine, pages 1-6, Jul 2024. URL: https://doi.org/10.29089/paom/188059, doi:10.29089/paom/188059. This article has 2 citations.

11. (brodszki2020europeansocietyfor pages 5-6): Nicholas Brodszki, Ashley Frazer-Abel, Anete S. Grumach, Michael Kirschfink, Jiri Litzman, Elena Perez, Mikko R. J. Seppänen, Kathleen E. Sullivan, and Stephen Jolles. European society for immunodeficiencies (esid) and european reference network on rare primary immunodeficiency, autoinflammatory and autoimmune diseases (ern rita) complement guideline: deficiencies, diagnosis, and management. Journal of Clinical Immunology, 40:576-591, Feb 2020. URL: https://doi.org/10.1007/s10875-020-00754-1, doi:10.1007/s10875-020-00754-1. This article has 117 citations and is from a domain leading peer-reviewed journal.

12. (sullivan2026complementdeficiencies pages 1-45): Kathleen E. Sullivan. Complement Deficiencies, pages 693-713. Springer Nature Switzerland, Jan 2026. URL: https://doi.org/10.1007/978-3-032-01547-1_9, doi:10.1007/978-3-032-01547-1_9. This article has 5 citations.

13. (khalil2023thefirstcase pages 1-2): Sally Mahgoub Khalil, Sami Aqel, Dalal Sideeg Mudawi, Hassan Mobayed, and Maryam Ali Al-Nesf. The first case report of complement component 7 deficiency in qatar and a 10-year follow-up. Frontiers in Immunology, Oct 2023. URL: https://doi.org/10.3389/fimmu.2023.1253301, doi:10.3389/fimmu.2023.1253301. This article has 2 citations and is from a peer-reviewed journal.

14. (hodeib2020humangeneticsof pages 1-2): Stephanie Hodeib, Jethro A. Herberg, Michael Levin, and Vanessa Sancho-Shimizu. Human genetics of meningococcal infections. Human Genetics, 139:961-980, Feb 2020. URL: https://doi.org/10.1007/s00439-020-02128-4, doi:10.1007/s00439-020-02128-4. This article has 40 citations and is from a peer-reviewed journal.

15. (lizamamunoz2025descriptionandphenotype pages 1-2): Asier Lizama-Muñoz, Juan Francisco Gutiérrez-Bautista, Monica Bernal, and Miguel Ángel López-Nevot. Description and phenotype of a novel c5 gene mutation and a novel combination: family report and literature review. Frontiers in Immunology, Jul 2025. URL: https://doi.org/10.3389/fimmu.2025.1605903, doi:10.3389/fimmu.2025.1605903. This article has 0 citations and is from a peer-reviewed journal.

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