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Complement C2 deficiency
This condition makes individuals more vulnerable to certain infections and can increase the risk of developing autoimmune diseases. It is a lifelong condition affecting the immune system's ability to fight off pathogens and clear cellular debris.
Overview
Complement C2 deficiency is a rare genetic condition in which the body does not produce enough of a specific protein called C2, which is part of the complement system. The complement system is involved in the immune response, helping the body to fight off infections and clear away damaged cells. When C2 is deficient, this part of the immune system cannot function properly, leading to increased susceptibility to certain illnesses [PMID:31536391].
This condition is one of the more common types of complement deficiencies, particularly in populations of European descent, affecting approximately 1 in 20,000 individuals [PMID:33672729]. It is present from birth and affects individuals throughout their lives, requiring careful management to minimise health complications. The severity of the condition can vary between affected individuals.
Symptoms & clinical features
Individuals with Complement C2 deficiency often experience recurrent bacterial infections, particularly those caused by encapsulated bacteria such as *Streptococcus pneumoniae* and *Haemophilus influenzae*. These infections commonly affect the respiratory tract (leading to pneumonia or bronchitis) and the ears.
Another significant feature is an increased risk of developing autoimmune diseases, with systemic lupus erythematosus (SLE) being the most frequently observed. Symptoms of SLE can include joint pain, skin rashes (such as the 'butterfly' rash on the face), fatigue, and kidney problems. Rarely, individuals might experience less common infections or other autoimmune manifestations [PMID:31536391].
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Affected organs
The immune system is primarily affected, leading to a reduced ability to defend against pathogens throughout the body. Organs commonly impacted due to recurrent infections include the lungs and ears. The skin and joints can be affected in individuals who develop autoimmune conditions like systemic lupus erythematosus (SLE). Kidneys and the central nervous system can also be involved in more severe cases of SLE.
Risks & severity
The severity of Complement C2 deficiency can vary. While some individuals may experience only mild, infrequent infections, others can have recurrent severe bacterial infections, particularly early in life. A major risk is the development of autoimmune diseases; research suggests that around 10-20% of individuals with C2 deficiency will develop systemic lupus erythematosus (SLE) [PMID:33672729]. Other autoimmune conditions are possible but less common. The age of onset for infections can be in childhood, while autoimmune conditions may develop later, often in adolescence or adulthood.
Untreated or poorly managed infections in individuals with C2 deficiency can lead to serious complications, including sepsis or meningitis. Early diagnosis and appropriate medical care are important for managing these risks and improving health outcomes.
Genetic causes
Complement C2 deficiency is caused by pathogenic variants in the C2 gene. This gene provides instructions for making the C2 protein, a crucial component of the complement system. The complement system is a complex network of proteins that form part of the innate immune system, acting as a first line of defence against pathogens. It also helps to clear immune complexes and dead cells from the body.
When a pathogenic variant occurs in the C2 gene, it leads to the production of a non-functional or absent C2 protein. Without sufficient functional C2 protein, the complement cascade, particularly the classical and lectin pathways, cannot be effectively activated. This impaired function compromises the body's ability to initiate an effective immune response, making individuals more susceptible to certain types of infections and contributing to the development of autoimmune conditions [PMID:31536391].
- C2 complement C2
Inheritance pattern
Complement C2 deficiency follows an autosomal recessive inheritance pattern. This means that an individual must inherit two copies of the altered C2 gene - one from each parent - to develop the condition. People who inherit only one copy of the altered gene are known as carriers. Carriers typically do not show symptoms of the condition themselves but can pass the altered gene on to their children.
If both parents are carriers, there is a 25% chance with each pregnancy that their child will inherit two altered gene copies and develop Complement C2 deficiency. There is a 50% chance the child will be a carrier, and a 25% chance the child will inherit two normal gene copies and not be affected or a carrier.
When both parents are carriers, each child has a 25% chance of being affected, 50% of being a carrier, and 25% of being unaffected.
Diagnosis & testing
Diagnosis of Complement C2 deficiency typically begins with clinical suspicion due to a history of recurrent infections or early-onset autoimmune disease, particularly systemic lupus erythematosus. Initial laboratory tests often include measuring complement protein levels in the blood, such as C2 and CH50 (total haemolytic complement activity). A significantly reduced or absent C2 level, along with very low CH50 activity, strongly suggests a diagnosis.
Confirmation of the diagnosis is usually achieved through genetic testing, which looks for pathogenic variants in the C2 gene. In the UK, genetic testing for C2 deficiency often falls under the NHS Genomic Medicine Service (GMS) pathways. Referrals for genetic testing and counselling are typically made by a clinical immunologist, rheumatologist, or a general practitioner to a clinical genetics service. This testing helps to confirm the condition and can inform cascade screening for family members who may be carriers.
Management & lifestyle
Management of Complement C2 deficiency focuses on preventing and promptly treating infections, and carefully monitoring for and managing autoimmune conditions. Individuals are often advised to have regular vaccinations, including the pneumococcal vaccine and annual flu jabs, to reduce the risk of common infections. Antibiotics may be prescribed for specific infections, and some individuals with severe or recurrent infections may require long-term prophylactic antibiotics.
For those who develop autoimmune conditions like SLE, management will involve specialists such as rheumatologists and will follow standard treatment protocols for these conditions. Regular clinical follow-ups with an immunologist are important to monitor immune function and overall health. Genetic counselling is also recommended for affected individuals and their families to understand the inheritance pattern and implications for family planning.
UK care pathway
In the UK, individuals suspected of having Complement C2 deficiency would typically be referred to specialists within the NHS, such as clinical immunologists or rheumatologists, who can initiate diagnostic investigations. Genetic testing is provided through the NHS Genomic Medicine Service (GMS), often following referral from a specialist clinic to a regional clinical genetics service. This ensures access to appropriate testing and genetic counselling to discuss the implications for the patient and their family. The relevant NHS Genomic Test Directory R-codes ensure testing is standardised and accessible.
Frequently asked questions
What is the complement system?
The complement system is a vital part of your immune system. It's made up of many proteins that work together to help identify and kill harmful bacteria and viruses, and to clear away damaged cells from your body. It acts as a rapid first line of defence against infections.
How serious is Complement C2 deficiency?
The seriousness can vary. Some people have mild symptoms with occasional infections, while others experience more severe, recurrent infections or develop autoimmune diseases like lupus. Careful medical management and monitoring are key to managing the condition effectively.
Can Complement C2 deficiency be cured?
No, Complement C2 deficiency is a lifelong genetic condition and cannot be cured. However, the symptoms and associated risks, such as infections and autoimmune diseases, can be managed effectively with appropriate medical care and surveillance.
What should individuals with C2 deficiency do to stay healthy?
Individuals should work closely with their doctors, typically an immunologist, to develop a personalised care plan. This often includes staying up-to-date with vaccinations, seeking prompt treatment for infections, monitoring for autoimmune symptoms, and avoiding known risk factors for illness.
Are my children at risk if I have Complement C2 deficiency?
Because it's an autosomal recessive condition, if you have Complement C2 deficiency (meaning you have two altered gene copies), your children will inherit one altered copy from you. Whether they also have the condition depends on the gene copy they inherit from their other parent. Genetic counselling can provide specific risk information for your family.
References
- Leddy JP, Griggs RC, Klemperer MR. Hereditary complement (C2) deficiency with dermatomyositis. The American journal of medicine. 1975. PMID: 1090155
- Cui J, Roark CM, Domínguez-Pinilla N. Primary Amoebic Meningoencephalitis caused by Complement C2 Deficiency. medRxiv : the preprint server for health sciences. 2026. PMID: 41674601
- Truedsson L, Alper CA, Awdeh ZL. Characterization of type I complement C2 deficiency MHC haplotypes. Strong conservation of the complotype/HLA-B-region and absence of disease association due to linked class II genes. Journal of immunology (Baltimore, Md. : 1950). 1993. PMID: 7901282
- Johnson CA, Densen P, Hurford RK Jr. Type I human complement C2 deficiency. A 28-base pair gene deletion causes skipping of exon 6 during RNA splicing. The Journal of biological chemistry. 1992. PMID: 1577763
- Martini PG, Cook LC, Alderucci S. Recombinant human complement component C2 produced in a human cell line restores the classical complement pathway activity in-vitro: an alternative treatment for C2 deficiency diseases. BMC immunology. 2010. PMID: 20727163
- Zhu ZB, Atkinson TP, Volanakis JE. A novel type II complement C2 deficiency allele in an African-American family. Journal of immunology (Baltimore, Md. : 1950). 1998. PMID: 9670930
- Sibbald RG, Eady RA, Kobza-Black A. Cold urticaria and complement C2 deficiency. The British journal of dermatology. 1979. PMID: 465334
- Lipsker DM, Schreckenberg-Gilliot C, Uring-Lambert B. Lupus erythematosus associated with genetically determined deficiency of the second component of the complement. Archives of dermatology. 2000. PMID: 11115162