The complement system is made up of a network of proteins that involve the immune system and inflammation. Decreased levels of various components of complement -- C3, C4, or CH50 -- can be seen in lupus. Other diseases that involve inflammation of the blood vessels -- called vasculitis -- can also have decreased levels of complement in the blood. In some cases of lupus, particularly those with kidney disease, complement levels are used to follow the illness because they may rise and fall according to the activity of the disease.