Protein S deficiency in lupus erythematosus secondary to hereditary angio‐oedema

Summary We report a child with lupus erythematosus secondary to type 2 hereditary angio‐oedema (HAE). Reduced levels of free protein S were found. Total protein S, and C4 binding protein (C4bp) were normal, and there was no evidence of anticardiolipin antibodies or lupus anticoagulant. We postulate that reduced levels of C4, secondary to C1 inhibitor deficiency, resulted in increased binding of protein S to C4bp, leading to reduced levels of free protein S and thus functional deficiency of protein S.