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C1 Esterase Inhibitor

C1 esterase inhibitor is a serum alpha-2 globulin and a member of the serpin family of protease inhibitors that is synthesized by the liver. Its physiologic function is inhibition of the catalytic subunits of the first component of the classic complement pathway (C1r and C1s). Deficiency of C1 esterase inhibitor results in the inappropriate activation of C1 and generation of C2 kinin. The latter molecule increases vascular permeability and is believed to be the mediator of the angioedema observed in patients with C1 esterase inhibitor deficiency. Edema involves the subcutaneous tissue, gastrointestinal tract and respiratory tract.

There are two forms of C1 esterase inhibitor deficiency. The inherited form is usually detected in the first or second decade of life and has an autosomal dominant pattern of inheritance. The acquired form primarily affects adult or elderly patients with autoimmune or lymphoproliferative disorders.

In the inherited form of C1 esterase inhibitor deficiency, there is either a quantitative or qualitative defect in the synthesis of the C1 esterase inhibitor protein. The more common form of hereditary angioneurotic edema (85% of cases) is due to an absolute decrease in the synthesis of the C1esterase inhibitor. The less common form (15% of cases) is due to production of normal quantities of a functionally deficient protein. In both hereditary subtypes, C1 activation proceeds unabated, resulting in normal levels of C1q. Levels of C2 and C4 are decreased because of the uncontrolled activity of C1s.

Patients with the acquired form of C1 esterase inhibitor deficiency produce immune complexes that consume large amounts of C1q and C1 esterase inhibitor, resulting in quantitative and functional deficiency of the C1 esterase inhibitor and C1q. C2 and C4 levels are also decreased.

Differentiation of Acquire and Hereditary Deficiencies

Type of Deficiency

C1q Levels

C2 & C4 Levels







Reference range is 18 – 40 mg/dL.

Specimen requirement is one lavender (EDTA) tube of blood.

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