Additional Information

The complement system is an integral part of the nonspecific antigen immune defenses. It can be activated via two reaction pathways; the classical pathway, which is triggered primarily by cell-bound immune complexes, and the alternative pathway, which is activated primarily by foreign bodies such as microorganisms. The complement component C3 is a key protein in both reaction pathways. Complement activation is associated with consumption of components C3 and/or C4 so that a reduction in their concentrations is diagnostically useful.

Diminished serum concentrations of C3 are observed primarily in active systemic lupus erythematosus (SLE), in forms of membrane proliferative glomerulonephritis and in immune complex diseases (serum sickness). In the case of SLE the serum concentrations of the complement factors reflect the activity of the disease. Diminished C3 values occur in acute glomerulonephritis and in membrane proliferative glomerulonephritis. Complement component C3 reacts as an acute-phase protein and may therefore show elevated serum concentrations in patients with inflammatory diseases. Hereditary deficiency states of complement C3 have been reported.

Over time C3 converts to the functionally inactive but stable and immunoreactive fragment C3c. This will happen in vivo and in vitro. The detection of the cleavage fragment is the method of choice in investigating complement activation.

LINKS: Reference Range