Activation of the complement cascade in a patient deficient in C3:
(1) fails to generate major complement opsonin and
(2) fails to activate the membrane attack-complex pathway.
Gene location: chromsome 19
Inheritance: autosomal recessive
Clinical features:
(1) recurrent pyogenic bacterial infections, typically starting during childhood
(2) vasculitic rash that often appears during a pyogenic infection
(3) membranoproliferative glomerulonephritis (immune complex-mediated)
(4) variable arthralgias
The pyogenic infections typically involve encapsulated organisms such as:
(1) Streptococcus pneumoniae
(2) Hemophilus influenzae
(3) Neisseria meningitidis
Laboratory findings:
(1) low serum levels of C3
Differential diagnosis:
(1) acquired deficiency of C3
