CD200 and systemic lupus erythematosus: Furthermore, the serum CD200 level negatively correlated with the serum complement 3 level (r = -0.45, P = 0.014; Figure 2B), but not the Systemic Lupus Erythematosus Disease Activity Index score or the levels of serum B-cell activating factor belonging to the TNF family, IL-6, IFNα, or anti-dsDNA (P > 0.05; see Additional file 3).