C4A and systemic lupus erythematosus: This study aims to assess the role of vitamin D on systemic lupus erythematosus (SLE) patients and its effects on systemic lupus erythematosus disease activity index (SLEDAI), anti-double-stranded DNA (anti-dsDNA), C3, C4, and fatigue in patients with SLE. A systemic search was conducted using three electronic databases, i.e., PubMed/Medline, Cochrane Library, and Google Scholar.