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 C4 belongs to the classical pathway of complement activation. 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 C4 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. Isolated diminished levels of C4 can occur in heredity angioneurotic oedema and in cases of cryoglobulinaema. Complement component C4 reacts as an acute-phase protein and may therefore show elevated serum concentrations in patients with inflammatory diseases. Hereditary deficiency states of complement C4 have been reported.

C4 is a relatively fragile protein, especially in the presence of Ca++. Activation of complement and splitting of C4 can occur at room temperature, or colder, in many samples with or without cryoglobulins.