C4A and systemic lupus erythematosus: Negatively correlated with creatinine and anti-dsDNA concentration and positively correlated with C3 and C4 levels.As a biomarker in the diagnosis of SLE, the diagnostic efficiency of MMP9 promoter methylation level for SLE was 0.839;Destructed autoantigens captured in immune complexes, supplemented the complement system in immune complexes clearance.