Hypocomplementemia, specifically low C4 levels, was associated with an increased risk of autoimmune hemolytic anemia (OR = 5.88, 95% CI: 1.57–22.04, p = 0.009), whereas reduced C3 levels were significantly associated with the presence of lupus nephritis (p = 0.017). The gene discussed is C4A; the disease is autoimmune hemolytic anemia.