Low complement C3 and C4 may be due to decreased ability of complement synthesis and production, common diseases include chronic active hepatitis, cirrhosis, hepatocellular necrosis, etc. It may also be due to excessive complement depletion or loss, such as various acute and chronic glomerulonephritis, systemic lupus erythematosus, rheumatoid arthritis, extensive burns can also lead to excessive complement depletion, there may also be congenital genetic diseases that lead to complement deficiency, and malnutrition in children that leads to insufficient complement synthesis material can also lead to lower c3c4.