Complement is an independent and specific protein system of the body’s immune system that is mainly present on the surface of serum, body fluids and tissue cells. Its roles are bacteriolytic and cytolytic, regulatory phagocytosis, immune adhesion, and inflammatory mediator. The decrease of complement can be caused by the following reasons: decreased capacity of complement synthesis, excessive consumption or loss, insufficient synthesis of raw materials, congenital complement deficiency. Clinically, c3c4 is usually considered in combination, and its low level mainly indicates the presence of some tissue inflammation, reduced immunity and other aspects of the disease. In the acute or exacerbation phase of the disease, a decrease in c3c4 will occur, but when the inflammation or the disease is in remission, complement will return to normal values. C3 normal reference value is 0.9~1.8g/L. The decrease is common in: chronic active hepatitis, cirrhosis and other liver diseases; chronic glomerulonephritis caused by immune complexes, glomerulonephritis after acute streptococcal infection, lupus nephritis and other kidney inflammation; recurrent infections, burns, systemic lupus erythematosus and other acute destruction and severe wasting diseases. The normal reference value of C4 is 0.1-0.4g/L. Its decrease is seen in nephritis caused by immune complexes, systemic lupus erythematosus, viral infection, lupus syndrome, cirrhosis, hepatitis, multiple sclerosis, rheumatoid arthritis, IgA nephropathy, subacute sclerosing allodynia, etc.