The C1 inhibitor deficiency. A review.

C1 inhibitor (C1I), a member of the serine protease inhibitor superfamily, is the principal regulator of the activation classical pathway of complement by reducing the proteolytic activity of activated C1r and C1s. A deficiency of active C1 inhibitor is the most commonly identified genetic defect of the complement system. It is associated with a pathology called angioedema. There are three forms of hereditary angioedema. The first type is characterized by an insufficient production of a normal protein. The two other forms are characterized by the presence of an abnormal C1 inhibitor protein. Moreover a reduction of functional C1 inhibitor may also be acquired. There are two types of acquired angioedema, a form associated with malignancy (B cell lineage, breast cancer, ...) and an autoimmune form. Angioedema manifests itself by attacks of swelling of the extremities, face, trunk, airways, or abdominal viscera, occurring spontaneously or as a result of trauma. Three main categories of substances have been proposed for the treatment of C1 deficiencies: the androgens, the antifibrinolytics and fresh plasma or purified C1 inhibitor. To distinguish between the different forms of C1 inhibitor deficiencies, it is necessary to determine the amount of C1 inhibitor protein and the level of its functional activity. Several methods for the determination of C1 inhibitor have been proposed: titrimetric and spectrophotometric assays, inhibition of complement haemolytic activity, radioimmunoassay, enzyme-linked immunosorbent assay, ...), in order to improve the diagnosis and the treatment of angioedema.