Hereditary Deficiency of the Second Component of Complement (C'2) in Man: Further Observations on a Second Kindred

A kindred including a homozygous C′2 deficient individual and three heterozygous C′2 deficient individuals has been reinvestigated. The deficiency appears to be transmitted as an autosomal recessive trait in which heterozygous individuals can be detected. Serum complement activity is decreased as measured by hemolytic activity, bactericidal capacity and immune adherence. The deficiency is due to a selective lack of C′2 activity and full serum complement activity is restored by the addition of highly purified C′2. The induction of vasopermeability activity by C′1 esterase is markedly decreased in C′2 deficient individuals. The defect in the second component of complement appears to be the result of defective protein biosynthesis rather than the production of a serum inhibitor or an active but unstable form of C′2.