A complement test is a blood test that measures the activity of proteins in part of blood known as serum. These group of proteins make up the complementary system which is a part of human immune system. The complement system helps antibodies to fight infections and destroy substances that are alien to the body. The complement system is also activated when body makes antibodies to fight with its own issues that are considered to be foreign by the body. This is the case of autoimmune diseases. This test can measure the progress of people undergoing treatment of autoimmune diseases. The test can see how advanced the proteins of these diseases by the activity of complement proteins in the blood.