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).

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

Mannose-binding lectin (MBL) deficiency, defined as MBL protein level of less than 100 ng/ml, is present in about 5% of people of European descent and in about 10% of sub-Saharan Africans. Most MBL-deficient adults appear healthy, but low levels of MBL are associated with increased risk of infection in toddlers, in cancer patients undergoing chemotherapy, and in organ-transplant patients receiving immunosuppressive drugs, particularly recipients of liver transplants (review by Degn et al., 2011). MBL is a soluble molecule that can activate the lectin pathway of the complement system; deficiency may thus lead to defects in the complement system (summary by Garcia-Laorden et al., 2008). Genetic Heterogeneity of Lectin Complement Activation Pathway Defects See also LCAPD2 (613791), caused by variation in the MASP2 gene (605102) on chromosome 1p36, and LCAPD3 (613860), caused by variation in the FCN3 gene (604973) on chromosome 1p36.

C3 glomerulopathy (C3G) is a complex ultra-rare complement-mediated renal disease caused by uncontrolled activation of the complement alternative pathway (AP) in the fluid phase (as opposed to cell surface) that is rarely inherited in a simple mendelian fashion. C3G affects individuals of all ages, with a median age at diagnosis of 23 years. Individuals with C3G typically present with hematuria, proteinuria, hematuria and proteinuria, acute nephritic syndrome or nephrotic syndrome, and low levels of the complement component C3. Spontaneous remission of C3G is uncommon, and about half of affected individuals develop end-stage renal disease (ESRD) within ten years of diagnosis, occasionally developing the late comorbidity of impaired visual acuity.