Frequently Asked Questions (FAQs)
The complement system is a major component of innate and adaptive immunity; upon activation, the complement results in the formation of the membrane attack complex (MAC) which releases peptides called anaphylatoxins. About 90% of complement components are synthesized in the liver and are acute-phase proteins. C4 is one of the activation proteins of the classic pathway. Decreased C4 levels Acquired autoimmune disorders, active phase of lupus erythematosus and rheumatoid arthritis. An undetectable C4 level (with normal C3) suggests a congenital C4 deficiency. Increased C4 levels Autoimmune hemolytic anemia.