Hereditary and Acquired Angioedema (Acquired C1 Inhibitor Deficiency) By James Fernandez, MD, PhD, Cleveland Clinic Lerner College of Medicine at Case Western Reserve University Reviewed / Revised Oct 2022 View Patient Education Symptoms and Signs Diagnosis Treatment Prevention Key Points Topic Resources Complement activation pathways Hereditary Angioedema Hereditary angioedema and acquired angioedema (acquired C1 inhibitor deficiency) are caused by deficiency or dysfunction of complement 1 (C1) inhibitor, a protein involved in the regulation of the classical and lectin complement activation pathways, and of the kinin, clotting, and fibrinolytic pathways.