Decreased C4 complement factor levels (8.5 ± 3.7, range 5.0–12.0 mg/dL) were found in nine patients (11%), three of them in the setting of autoimmune disease (Sjögren syndrome, autoimmune haemolytic anaemia (AIHA), and immune thrombocytopenia (ITP)). Here, C4A is linked to autoimmune hemolytic anemia.