A rare genetic disorder with an autosomal recessive pattern of inheritance. It is caused by the ineffective or decreased biosynthesis of the fifth complement component, C5. C5 deficiency may also be acquired acutely post-infection. If C5 is adequately synthesized, its rapid depletion may result in a functional deficiency. Clinical signs of the inherited deficiency present within the second decade of life and are consistent with the signs of recurrent systemic infection. Deficiency of serum C5 and its major cleavage product, C5b, a component of the membrane attack complex, increases susceptibility to Neisserial infections.

Any classic complement early component deficiency in which the cause of the disease is a mutation in the C6 gene.

A form of hereditary angioedema characterized by acute edema in subcutaneous tissues, viscera and/or the upper airway.

Concentration of the complement component C4a in the blood circulation below the lower limit of normal.

C1q deficiency (C1QD) is a rare autosomal recessive disorder characterized by recurrent skin lesions, chronic infections, and an increased risk of autoimmune diseases, particularly systemic lupus erythematosus (SLE; see 152700) or SLE-like diseases. It has also been associated with chronic glomerulonephritis and renal failure. C1q deficiency presents in 2 different forms, absent C1q protein or presence of a dysfunctional molecule (summary by Topaloglu et al., 1996 and Vassallo et al., 2007). For a discussion of genetic heterogeneity of C1q deficiency, see 613652.