Health condition
Complement deficiencies
The complement system is a cascade of blood proteins that punches holes in bacteria and clears immune debris; inherited deficiencies produce two signature problems - recurrent neisserial (meningococcal) infection and lupus-like autoimmunity - and are screened with two simple functional tests.
Why trust this guide
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Editorial team
Written by our editorial team from the UK sources listed on each page.
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Sourced from guidance
Checked against current NHS and UK specialist society guidance at the time of review.
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Current for 2026
Reviewed on 2026-09-27 with the next review scheduled for 2027-09-27.
Key facts
The essentials at a glance.
The symptoms to recognise, when to seek help, which specialist to see, and what treatment usually involves.
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Screening is easy and underused: CH50 (classical pathway) and AP50 (alternative pathway) functional assays detect almost all significant deficiencies
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Terminal pathway (C5-C9) and properdin deficiencies specifically invite meningococcal disease - often recurrent, or from unusual serogroups
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Early classical pathway deficiencies (C1q, C4, C2) predispose strongly to lupus-like disease and infections
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Anyone with two meningococcal episodes, one unusual-serogroup episode, or a striking family history should be tested
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Management: full meningococcal (ACWY and B) vaccination, low-threshold antibiotics, sometimes prophylaxis - and family screening
Understanding the condition
What it is and what happens next.
What causes it, how it is diagnosed, and the treatment routes available in the UK.
What it is and how it feels
Complement works as a chain reaction, so a single missing protein silences everything downstream. Nature runs the experiment for us: people missing terminal components handle most infections normally but meet meningococcus repeatedly - the membrane-attack complex is precisely the weapon that kills neisseria - while those missing early components fail to clear apoptotic debris and immune complexes, developing photosensitive rashes, arthritis and kidney disease that look like lupus, often with atypically negative or unusual antibody patterns. Both stories hide in plain sight for years.
Diagnosis and treatment routes
The functional screens cost little: a normal CH50 and AP50 together effectively exclude inherited deficiency, while an absent result localises the missing protein for confirmation. Consequences are practical - aggressive meningococcal vaccination, rescue antibiotics or prophylaxis, screening siblings, and informed lupus care - and increasingly relevant as complement-blocking drugs (which deliberately create this state) spread through medicine. After any second or unusual meningococcal infection, insist on complement testing; a private immunologist can typically arrange it within about two weeks.
Seeing a specialist
Who to see, and how quickly you can be seen.
The specialist for this condition is usually a consultant clinical immunologist. Your GP can refer you on the NHS, or you can arrange a private consultation directly.
A quiet reminder
This guide is for information, not medical advice.
Your GP knows your history and can tell you which parts apply to you. If in doubt, see them - and seek urgent care for any of the emergency symptoms described above.
Frequently asked
Common questions about Complement deficiencies.
Quick, plain-English answers to what people ask us most.
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What are complement deficiencies?
Complement deficiencies are a group of rare immune conditions in which one or more components of the complement system — a set of proteins that work together as part of the immune system's response to infection and inflammation — are missing or not functioning properly. Depending on which specific component is deficient, this can lead to increased susceptibility to certain infections, or, in some cases, an increased risk of autoimmune conditions. There are many different specific complement deficiencies, each with somewhat different clinical features.
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What causes complement deficiencies?
Most complement deficiencies are inherited, caused by genetic mutations affecting the production or function of a specific complement protein. Inheritance patterns vary depending on the specific deficiency, with some being autosomal recessive and others autosomal dominant. Genetic testing can often identify the specific deficiency and its inheritance pattern, which is relevant for family counselling.
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What symptoms do complement deficiencies cause?
Symptoms vary considerably depending on which specific complement component is deficient, but can include an increased susceptibility to certain bacterial infections (particularly meningococcal infections with deficiencies of certain later complement components), and, with deficiencies of some earlier complement components, an increased risk of autoimmune conditions such as lupus. Some deficiencies cause few or no symptoms and are found incidentally. Understanding which specific component is affected helps predict the likely pattern of associated risks.
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Are complement deficiencies dangerous?
This depends on the specific deficiency — some carry a significantly increased risk of serious infections, particularly meningococcal disease, which can be life-threatening if not promptly recognised and treated, while others cause milder or more variable clinical effects. Identifying the specific deficiency helps guide appropriate preventive measures, such as specific vaccinations and prompt treatment protocols for infections. A specialist immunology assessment can clarify the specific risks relevant to your particular deficiency.
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How are complement deficiencies managed?
Management typically includes specific vaccinations (particularly against meningococcal disease for relevant deficiencies), prompt and sometimes more aggressive treatment of any infections that do occur, and monitoring for associated autoimmune conditions where relevant to the specific deficiency. Some people are advised to carry specific information about their condition for use in emergency medical situations, given the potential severity of certain infections. A specialist immunology team typically coordinates this ongoing, tailored care.
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When should I see a specialist?
See a doctor if you have recurrent or unusually severe infections, particularly meningococcal infections, or a family history of complement deficiency, as specialist immunology assessment can identify the specific deficiency and guide appropriate preventive measures. NHS immunology referral can take several weeks to months; a private consultation is typically available within about 2 weeks for earlier specialist assessment.