Complement tests measure the quantity or activity of complement proteins in the blood. The complement system is composed of a set of circulating blood proteins that work together to promote protective immunity. Their principal role is to destroy bacteria and viruses. The nine main complement proteins are labelled C1 through C9. These proteins are assisted and regulated by several subcomponents, inhibitors, and regulators. …Complement tests measure the quantity or activity of complement proteins in the blood. The complement system is composed of a set of circulating blood proteins that work together to promote protective immunity. Their principal role is to destroy bacteria and viruses. The nine main complement proteins are labelled C1 through C9. These proteins are assisted and regulated by several subcomponents, inhibitors, and regulators. The complement system is part of the body’s innate immune system. Complement activation may be achieved in 3 different ways. These are termed classical, alternative or lectin pathways. However, the final product from all activation pathways is the same – the formation of the Membrane Attack Complex (MAC). Complement activation causes several things to happen:The MAC binds to the surface of each microorganism or abnormal cell that has been targeted for destruction. It creates a hole and causes the cell to rupture by letting the contents leak out – like piercing a water-filled balloon. It increases the leakiness of blood vessels, allowing infection-fighting white blood cells (WBCs) to move out of the bloodstream and into the tissues. It attracts WBCs to the site of the infection. It stimulates white blood cells such as macrophages and neutrophils to kill swallow and kill bacteria. It locks up the bacteria into cage like structures that dissolve in the bloodstream which are easily cleared. Complement proteins both promote and regulate these activities. Inherited or acquired deficiencies or abnormalities in one or more of the complement components may adversely affect the integrity and function of the immune system. Deficiencies may arise because of decreased production or increased consumption of one or more complement proteins. These tests measure the quantity or the function of complement proteins in the blood. Complement components may be measured individually and together to determine whether the system is functioning normally. C3 and C4 are the most frequently measured complement proteins. Total complement activity (CH50 or CH100, and AP50 or AP100) can be measured if a doctor suspects a deficiency that is not measured by C3 or C4. CH50/CH100 measures the function of the complete classical complement pathway, and AP50/100 measures the function of the complete alternative complement pathway. If this measurement is significantly reduced, then each of the nine different complement levels can be measured individually to look for hereditary or acquired deficiencies. In some rheumatologic disease C1q levels or antibodies are measured. Patients with a history of recurrent unexplained swelling may have C4 and C1 esterase inhibitor measured. See MoreSee Less
