|Classification and external resources|
|OMIM||217000 120820, 120900, 610102|
|DiseasesDB||1847 1869, 1873, 7384, 34381|
The disorders can be divided into two categories:
- Disorders of the proteins that act to inhibit the complement system (such as C1-inhibitor) can lead to an overactive response, causing conditions such as hereditary angioedema and hemolytic-uremic syndrome.
- Disorders of the proteins that act to activate the complement system (such as C3) can lead to an underactive response, causing greater susceptibility to infections.
Hypocomplementemia may be used more generally to refer to decreased complement levels while secondary complement disorder is to low complement levels that are not directly due to a genetic cause but secondary to another medical condition.
The total hemolytic complement complement CH50 level in the blood will be low or undetectable with complement deficiencies.
Individual complement levels can be used to distinguish conditions:
- Systemic lupus erythematosus is associated with low C3 and C4
- Membranoproliferative glomerulonephritis causes low C3, but normal C4
- Deficiencies of the terminal complement components are inherited in an autosomal recessive manner and cause increased susceptibility to infections by Neisseria.
- Properdin deficiency is an X-linked disorder that also causes susceptibility to neisserial infections.
- C1-inhibitor deficiency or hereditary angioedema will have low C4 with normal C1 and C3 levels. 
Vaccinations for encapsulated organisms is crucial for preventing infections in complement deficiencies.
- Sherwood L. Gorbach; John G. Bartlett; Neil R. Blacklow (2004). Infectious diseases. Lippincott Williams & Wilkins. pp. 11–. ISBN 978-0-7817-3371-7. Retrieved 30 May 2010.
- Sjöholm AG, Jönsson G, Braconier JH, Sturfelt G, Truedsson L (2006). "Complement deficiency and disease: an update". Mol. Immunol. 43 (1–2): 78–85. doi:10.1016/j.molimm.2005.06.025. PMID 16026838.
- "hypocomplementemia" at Dorland's Medical Dictionary
- Harrison's Principles of Internal Medicine, 17th edition 
- Bruce L. Zuraw, M.D. (September 4, 2008). "Hereditary Angioedema". N Engl J Med 2008 359: 1027-1036. doi:10.1056/NEJMcp0803977. PMID 18768946.