Assays / Protein Reference Units / Complement C2 and C5-C9

Clinical use:

Clinical conditions associated with monocomponent complement deficiencies:

• C1q SLE, Glomerulonephritis, Infections
• C2 SLE, Glomerulonephritis, Infections - often no clinical abnormality.
• C4 SLE
• C3 Neisserial infection, Glomerulonephritis
• C5 Neisserial infection, SLE
• C6 Neisserial infection, SLE
• C7 Neisserial infection, SLE, Rheumatoid arthritis
• C8 Neisserial infection, SLE
• C9 No evidence of disease.
• D recurrent pyogenic infection, meningitis, septicaemia
• P recurrent pyogenic infection, meningitis, septicaemia
• I recurrent pyogenic infection, meningitis, septicaemia
• H Haemolytic-uraemic syndrome, glomerulonephritis

C2 is low in conditions in which classical complement pathway activation has occurred, ie as a consequence of immune complex mediated activation. It is sometimes a more sensitive indicator of complement activation than either C3 or C4.

C2 null alleles are probably the most common genetic abnormality of the complement system (estimated gene frequency 1:100 to 1:500) and low concentrations may be due to heterozygous deficiency. Complete deficiency should result in an absent CH50. It is compatible with good health, but such individuals are more susceptible to immune complex disease, such as SLE.

C5, C6, C7, C8 and C9 are components of the membrane attack complex which directly results in cell lysis. Reduce, but measurable, concentrations of individual components of the MAC are seen in any ‘chronic’ activation of the complement cascade and are of little clinical significance, since when this occurs there are invariable similar changes in the C3 and C4 components.

The only clinical interest in this part of the complement sequence is in the rare occurence of homozygous genetic defects of any of the terminal components. All such patients will have an absent CH50. This leads to an increased tendency to infection with Neisseria organisms, in particular N.meningitidis and N.gonorrhoeae. Thus, while in the main, such individuals remain healthy, this genetic defect should be considered in any patient who has had more than one attack of meningitis, or in those with a family history of severe meningitis, or in someone with persistent or recurrent gonococcal septicaemia. The exception to this general rule is C9; patients with homozygous genetic defects of C9 usually have some CH50 activity. The defect is without clinical significance, since such individuals so far reported have all been healthy.

Sample requirement: Specimens for these analyses should only be sent after discussion with the PRU. For the quantitation of C2 alone, a single serum sample, despatched on the day of collection, will be adequate. For the investigation of the various proteins of the Membrane Attack Complex - C5 to C9 - it is necessary to separate the serum into three or four aliquots of 0.5 - 1.0 mL each. An EDTA or FUTHAN-EDTA plasma specimen (1 mL) should also be submitted.

Reference range:
Whilst all these components have been isolated and characterised, they are generally present in serum in concentrations of less than 0.1 g/L. Whereas mean or expected concentrations are recorded for many of the complement components, there is little international agreement in standardisation and results are often expressed as a percentage of a normal serum pool.

C2	10 - 30 mg/L
C5	80 - 150 mg/L
C6	65 mg/L
C7	55 mg/L
C8	80 mg/L
C9	80 mg/L

Centre offering this assay: Sheffield

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