A Family with Complement Factor I Deficiency

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Abstract
A family with inherited factor I deficiency is described. The proband was a 19-year-old Caucasian female with one episode of meningococcal meningitis and one episode of suspected septicaemia of unknown cause. Two obligate and two probable heterozygotes with factor I levels below the lower limit of the reference range were identified. None of these exhibited increased susceptibility to infectious diseases. The inheritance was autosomal codominant. In addition, molecular heterogeneity of factor H in plasma from the proband but not from any other family members was demonstrated by cross immunoelectrophoresis. The migration of factor H component of fast electrophoretic mobility was retarded by antibodies to C3c and C3d, suggesting the presence of a fluid-phase complex between factor H and excess C3b generated by the uncontrolled activity of the amplification loop.