Complement deficiency is an immunodeficiency of absent or suboptimal functioning of one of the complement system proteins. Because of redundancies in the immune system, many complement disorders are never diagnosed. Some studies estimate that less than 10% are identified. Hypocomplementemia may be used more generally to refer to decreased complement levels, while secondary complement disorder means decreased complement levels that are not directly due to a genetic cause but secondary to another medical condition. Signs and symptoms
The following symptoms (signs) are consistent with complement deficiency in general:

Complications
Vaccinations for encapsulated organisms (e.g., Neisseria meningitidis and Streptococcus pneumoniae)  is crucial for preventing infections in complement deficiencies. Among the possible complications are the following:

Deficiencies of the terminal complement components increases susceptibility to infections by Neisseria. Causes
The cause of complement deficiency is genetics (though cases of an acquired nature do exist post infection). The majority of complement deficiencies are inherited as autosomal recessive conditions, while properdin deficiency occurs through X-linked inheritance. MBL deficiency can be inherited by either manner. Inherited
Properdin deficiency is an X-linked disorder that also causes susceptibility to Neisseria infections. C1-inhibitor deficiency or hereditary angioedema will have low C4 with normal C1 levels. Acquired
Acquired hypocomplementemia may occur in the setting of bone infections (osteomyelitis), infection of the lining of the heart (endocarditis), and cryoglobulinemia. Systemic lupus erythematosus is associated with low C3 and C4. Membranoproliferative glomerulonephritis usually has low C3. Mechanism
The mechanism of complement deficiency consists of:

C2: In regard to C2 deficiency, about 5 different mutations in the C2 gene are responsible.
Epidemiology
C2 deficiency has a prevalence of 1 in about 20,000 people in Western countries. See also
Paroxysmal nocturnal hemoglobinuria

References
Further reading
Botto, Marina (1 January 2001). "Links between complement deficiency and apoptosis". Arthritis Research & Therapy. 3 (4): 207–210. doi:10.1186/ar301. ISSN 1478-6362. PMC 128896. PMID 11438036. Aghamohammadi, Asghar; Rezaei, Nima (2012). Clinical cases in primary immunodeficiency diseases a problem-solving approach. Berlin: Springer. ISBN 9783642317859. Retrieved 21 September 2016. == External links ==