Complement deficiency

Complement deficiency is an immunodeficiency of absent or suboptimal functioning of one of the complement system proteins.^[1]

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.^[2]

The term “hypocomplementemia” is a more general term used to describe decreased complement levels.^[3] The term “secondary complement disorder” is sometimes used to refer to low complement levels that are not directly due to a genetic cause but secondary to another medical condition.^[4]

These levels can be used to distinguish conditions:

• Systemic lupus erythematosus causes low C3 and C4
• Membranoproliferative glomerulonephritis causes low C3, but normal C4

• Paroxysmal nocturnal hemoglobinuria

Template:Lymphoid and complement immunodeficiency