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.

Patients with C7 deficiency have an increased susceptibility to recurrent bacterial infections, especially meningitis caused by Neisseria meningitidis (Nishizaka et al., 1996).

Patients with deficiency of C8 suffer from recurrent neisserial infections, predominantly with meningococcus infection of rare serotypes. Most such patients are discovered among those having their first episode of meningitis at ages older than 10 years (Ross and Densen, 1984). Two types of inherited C8 deficiency have been reported in humans: type I (613790), in which only C8 alpha (C8A, 120950) and C8 gamma (C8G; 120930) are deficient, and type II, in which only C8 beta is deficient (Marcus et al., 1982; Tedesco et al., 1983). The 2 types are clinically indistinguishable (Ross and Densen, 1984).