All complement proteins are acute phase reactants and rise rapidly during inflammatory episodes. Conversely, the rates of complement protein catabolism may greatly increase in various autoimmune diseases. Because complement component determinations represent a static measurement of the net concentrations that result from a dynamic balance between component synthesis and catabolism, serial quantitations 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 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). C4 levels may be decreased in hereditary and acquired angioedema, complement activation due to immune complex diseases, decreased synthesis due to liver disease, increased consumption in glomerulonephritis, systemic lupus erythematosus (SLE), rheumatoid arthritis, respiratory distress syndrome, autoimmune hemolytic anemia, cryoglobulinemia, and sepsis. Total congenital C4 deficiency is rare, but partial C4 deficiency is common. Partial and complete congenital C4 deficiencies have been associated with immune complex diseases, SLE, autoimmune thyroiditis, and juvenile dermatomyositis. Infections associated with C4 deficiency include bacterial or viral men.
C4 levels will be decreased in acquired autoimmune disorders, in active phase of lupus erythematosus, and in rheumatoid arthritis. An undetectable C4 level (with normal C3) suggests a congenital C4 deficiency. Levels will be increased in patients with autoimmune hemolytic anemia.
1. Manufacturer’s reagent package insert, Architect® Complement 4, ABBOTT Laboratories, Diagnostic Division, Abbott Park IL 60064 USA, November 2015. 2. http://www.mayomedicallaboratories.com (Retrieved: 01 Jan 2019)