C4A and systemic lupus erythematosus: Increased frequency of C4 deficiency phenotypes was reported in IgA nephropathy and Henoch-Schönlein purpura (HSP) (26), insulin-dependent diabetes mellitus (IDDM) (149), systemic lupus erythematosus (SLE) (150, 151), repeated infections (152), juvenile idiopathic arthritis patients (27).