This test measures the amount of C3 proteins in your blood. These proteins are part of your complement system. This is an important part of your immune system. It helps kill bacteria and viruses that cause disease. C3 protein is the most important and abundant protein in the complement system. It covers microbes to destroy them. The test measures complement C3 levels and how they compare with other parts of the complement system. With this information, your healthcare provider can diagnose and track treatment of certain diseases. One of the diseases that often causes abnormal C3 is systemic lupus erythematosus (SLE). Lupus is an autoimmune disorder.