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Unit Code 81835:
C2 Complement, Functional, Serum

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Useful For

The investigation of a patient with a low (absent) hemolytic

complement (CH[50]).

Clinical Information

The classic pathway of the complement system is composed of a

series of proteins that are activated in response to the presence of

immune complexes. This activation process results in the formation

of the lytic membrane attack complex, as well as the generation of

activation peptides that are chemotactic for neutrophils and that

bind to immune complexes and complement receptors. The absence

of early components (C1, C2, C4) of the complement cascade results

in the inability of immune complexes to activate the cascade. Patients

with deficiencies of the early complement proteins are unable to

generate lytic activity or to clear immune complexes.

 

Although rare, C2 deficiency is the most common inherited complement

deficiency. Homozygous C2 deficiency has an estimated prevalence

ranging from 1:10,000 to 1:40,000 (the prevalence of heterozygotes is

1 in 100 to 1 in 50). Half of the homozygous patients are clinically normal.

 

However, discoid lupus erythematosus or systemic lupus erythematosus

(SLE) occurs in approximately one-third of patients with homozygous

C2 deficiency. Patients with SLE and a C2 deficiency frequently have a

normal anti-ds DNA titer. Clinically, many have lupus-like skin lesions

and photosensitivity, but immunofluorescence studies may fail to

demonstrate immunoglobulin or complement along the epidermal-

dermal junction.

 

Other diseases reported to be associated with C2 deficiency include

dermatomyositis, glomerulonephritis, vasculitis, atrophodema,

cold urticaria, inflammatory bowl disease, and recurrent infections.

 

The laboratory findings that suggest C2 deficiency include a hemolytic

complement (CH[50]) of nearly zero, with normal values for C3 and C4.

Reference Values

25-47 unit/mL

Interpretation

Absent (or low) C2 levels in the presence of normal C3 and C4 values

are consistent with a  C2 deficiency.

 

Low C2 levels in the presence of low C3 and C4 values are consistent

with a complement-consumptive process.

 

Low C2 and C4 values in the presence of normal values for C3 is

suggestive of C1 esterase inhibitor deficiency.

Cautions

Absent (or low) C2 functional levels in the presence of normal C2

antigen levels should be replicated with a new serum specimen to

confirm that C2 inactivation has not occurred during shipping.

 

If requested not to reflex low C2 result specimens for C3 and C4

testing, the following comment will be reported; "C2 result is

decreased. This could be a result of an inherited C2 deficiency,

complement consumption, or C1 esterase inhibitor or deficiency.

Analysis of C3 and C4 would be required for further interpretation."

Clinical Reference

1.   Gaither TA, Frank MM:  Complement. In Clinical Diagnosis and

      Management by Laboratory Methods. Seventeenth edition.

      Edited by JB Henry. Philadelphia, PA, WB Saunders Company,

      1984, pp. 879-892

 

2.   Agnello V:  Complement deficiency states. Medicine 1978; 57:1-23

 

3.   Buckley D, Barnes L:  Childhood subacute cutaneous lupus

      erythematosus associated with homozygous complement 2

      deficiency. Pediatr Dermatol 1995; 12:327-330

Key