Clinical Information

All complement proteins are acute phase reactants and rise rapidly in concentrations rise rapidly during inflammatory episodes. Conversely, rates of complement protein catabolism may greatly increase in various autoimmune diseases. Because complement component determinations represent a static measurement of net concentrations that result from a dynamic balance between component synthesis and catabolism, serial sample quantifications are more clinically useful.

Complement promotes inflammation or tissue damage during the immune response and plays an important role in the pathogenesis of some diseases. In the latter situation, complement is often activated by an abnormal antibody (autoantibody), an immune complex, or by foreign material. Increased complement C4 levels are associated with acute phase reactions and certain malignancies. Decreased levels of C4 occur in individuals with congenital deficiency or immunologic diseases (where complement is consumed at an increased rate). Total congenital C4 deficiency is rare but partial C4 deficiency is common. C4 deficiencies have been associated with immune complex diseases, systemic lupus erythematosus, autoimmune thyroiditis, and juvenile dermatomyositis. Infections associated with C4 deficiency include bacterial or viral meningitis, Streptococcus and Staphylococcus sepsis, and pneumonia.