C3 and systemic lupus erythematosus: In patients with active SLE, the formation of immune complexes leads to activation of the classical pathway of the complement system, decreasing serum levels of complement C3 and C4 and increasing the complement split products C3b and C3dg as well as the C3dg:C3 and C3b:C3 ratios, which all correlate with disease activity [64].