Complement Deficiencies

Patient Education

What is complement?

Complements are a group of proteins in the blood that help to fight infection. The complement system includes at least 30 proteins of which the common ones are C1, C2, C3, C4, C5, C6, C7, C8, C9, Factor B, Factor D & C1 inhibitor.

How do complements fight infections?

Each complement protein has a different role to fight infection, but they act in a cascade, one protein activating the next like a domino effect. These complements are activated either by the interaction of antibodies with the germs (classical) or by binding of mannose-binding leptin to the sugars on the bacterial surface (leptin pathway) or by another pathway (alternative pathway). C3 coats the bacteria so that they can be easily killed by the neutrophils. C5 to C9 punch holes in the bacteria causing them to rupture and die. C3 and C5 increase blood supply at the site of infection and thus increase the number of neutrophils at the infection site. C1 inhibitor helps to regulate the complement cascade and prevents it from activating uncontrollably.

What are complement deficiencies?

Deficiency in any protein of the complement system can lead to recurrent infections. Complement deficiencies are part of primary immunodeficiencies. Common complement deficiencies are deficiency of C3, deficiency of C2, deficiency of C5, and deficiency of C1 inhibitor.

What are the symptoms of complement deficiencies?

Patients with C3 deficiencies are prone to get recurrent bacterial infections. They are also more prone for get autoimmune diseases such as systemic lupus erythematosus (SLE) or juvenile rheumatoid arthritis. Patients with a deficiency of C5, C6, C7, C8, or C9 are prone to infection by a particular bacterium - Neisseria meningitides (it causes meningitis). Patients with a deficiency of C1 inhibitor are prone to get hereditary angioedema. (Hereditary angioedema is a condition that causes swelling of skin or the airways leading to breathing problems).

How is the diagnosis of complement deficiency made?

CH50 is the commonest test done to check the integrity of the complement cascade. Testing of individual components (their quantity and their ability to function properly) is done in special laboratories. CH50 is absent if there is a defect in one complement component. The function of the complement can be tested by the AP50 test.

Do complement deficiencies run in families?

Most complement deficiencies occur due to defects in both copies of a gene (autosomal recessive). Thus parents are usually unaffected but each-one has one defective gene. Only if the child gets both defective genes, will he/she will get the disease. Thus, the chance of a child being affected where both parents have a defective gene is 25%.

In the case of properdin deficiency, since the genetic defect is present on the x-chromosome, it affects only the boys. C1 inhibitor deficiency requires only one copy of the gene to be abnormal (autosomal dominant). Thus a child of a parent who has hereditary angioedema has a 50% chance of getting the disease.

What is the treatment of complement deficiency?

Prompt treatment of infections and immunization with meningococcal vaccine (in patients with C5-C9 deficiency) is useful. Androgens such as danazol or oxandrolone can be useful to treat patients with hereditary angioedema. C1 inhibitor can also be administered to terminate an attack.