C1S and hereditary angioedema: Hereditary C1 esterase inhibitor deficiency (hereditary angioedema; HAE) is a rare (incidence 1 in 10,000 to 1 in 150,000) autosomal dominant inherited disease of the complement system characterised by the absence or dysfunction of the protein C1 esterase inhibitor (C1 INH), which regulates the complement, fibrinolytic, coagulation and kinin cascades [1].