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- Complement is a major component of the innate immune system.
- Consists of plasma and membrane proteins which mediate 3 pathways of cascading enzyme reactions
- Pathway activation leads to inflammatory and immune responses.
- Deficiencies can arise in any of the proteins, leading to loss of activity of the deficient protein as well as loss of function of proteins that follow in the cascade.
- Inherited deficiencies of the complement components may predispose individuals to infections and autoimmunity.
- Secondary/acquired deficiencies are much more common than inherited deficiencies and are most often caused by increased consumption by immune complexes.
|C1q,r,s, C2||Systemic lupus erythematosus (SLE), bacterial infections|
|C4||SLE, autoimmune disorders|
|C3||Severe infections with encapsulated bacteria (i.e., Haemophilus influenzae), glomerulonephritis, immune complex disease|
|Factor H, I||Secondary C3 deficiency, atypical hemolytic uremic syndrome|
|Properdin||Males with neisserial and sinopulmonary infections|
|Factor D||Neisserial infections|
|MBL, MASP||Infections with encapsulated bacteria, SLE, rheumatoid arthritis|
|C5, 6, 7, 8, 9||Disseminated neisserial infections|
|DAF, CD59||Paroxysmal nocturnal hemoglobinuria|
|C1 inhibitor||Hereditary angioedema (HAE)|
- Complement deficiency accounts for approximately 2% of all primary immune deficiencies.
- Homozygous C2 deficiency 1 in 10,000
- Borderline C4 in 1–3% of Caucasian population
- C9 deficiency almost always found in people of Japanese descent
- C6 deficiency more common in African Americans
- Alternative pathway deficiencies (properdin, factor D) are rare.
- Properdin deficiency is X-linked.
- Most other complement deficiencies are autosomal recessive.
- C1 inhibitor deficiency is autosomal dominant.
- Heterozygotes are usually phenotypically normal.
- Classic complement pathway is activated when IgM or IgG antibodies bind to antigen.
- Lectin pathway is activated when a lectin such as mannose-binding lectin (MBL) binds to antigen.
- Alternative pathway does not need antibody or lectins to be activated.
- Main goal of all 3 pathways is to deposit C3b fragments on the target antigen to mark the target for immune response.
- Primary complement deficiencies are hereditary.
- Acquired deficiencies: accelerated consumption by immune complexes (most common), decreased hepatic production (less common), or loss through the urine (rare)