Complement deficiency

Complement deficiency is an immunodeficiency of absent or suboptimal functioning of one of the complement system proteins.

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.

Because there are redundancies in the immune system, many complement disorders are never diagnosed. A recent study estimated that less than 10% are identified.

Hypocomplementemia

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.

Epidemiology

C2 deficiency is prevalent in 120,000 people in Western countries. It occurs in about 1 in 10,000 persons.

See also

• Paroxysmal nocturnal hemoglobinuria