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The first component of complement (C1) is composed of 3 subunits designated as C1q, C1r, and C1s. C1q recognizes and binds to immunoglobulin complexed to antigen and initiates the complement cascade. Congenital deficiencies of any of the early complement components (C1, C2, C4) results in an inability to clear immune complexes. Inherited deficiency of C1 is rare.
Like the more common C2 deficiency, C1 deficiency is associated with increased incidence of immune complex disease (systemic lupus erythematosus, polymyositis, glomerulonephritis, and Henoch-Schonlein purpura). Low C1 levels have also been reported in patients with abnormal immunoglobulin levels (Bruton's and common variable hypogammaglobulinemia and severe combined immunodeficiency), and this is most likely due to increased catabolism.
The measurement of C1q is an indicator of the amount of C1 present.
Assessment of an undetectable total complement (CH50) level
Diagnosing congenital C1 (first component of complement) deficiency
Diagnosing acquired deficiency of C1 inhibitor
An undetectable C1q in the presence of an absent total complement (CH50) and normal C2, C3, and C4 suggests a congenital C1 (first component of complement) deficiency.
A low C1q in combination with a low C1 inhibitor and low C4 suggests an acquired C1 inhibitor deficiency.
This is a different assay than C1q binding, which is an assay for circulating immune complexes.
1. Frank MM: Complement in the pathophysiology of human disease. N Engl J Med 1987 June 11;316(24):1525-1530
2. Frank MM: Complement deficiencies. Pediatr Clin North Am 2000 December;47(6):1339-1354
3. Frigas E: Angioedema with acquired deficiency of the C1 inhibitor: a constellation of syndromes. Mayo Clin Proc 1989 October;64(10):1269-1275