ACR classification criteria for systemic lupus erythematosus: complement components.

Complement is involved in the pathogenesis of systemic lupus erythematosus (SLE) and has also a seemingly paradoxical protective role in the development of the disease. Low levels of components within the classical pathway of complement especially C1q, C4 and C3 have a high specificity for SLE diagnosis and should be considered as promising for inclusion in classification criteria of SLE.