Factor I deficiency and C3 nephritic factor: immunochemical findings and association with Neisseria meningitidis infection in two patients.

The complement system was examined in two patients with systemic Neisseria meningitidis infections, both of whom had reduced or nondetectable CH50 as analysed by both pathways. C3 measured by conventional technique revealed 19% anti-C3c-reactive protein in the plasma of patient 1 and 3% in patient 2. Patient 1 had circulating C3b but no detectable C3c, C3d, or C4d, whereas patient 2 had normal levels of C3c and C4d and strongly elevated levels of C3d. Factor B analysis revealed no demonstrable native factor B and small amounts of Bb in patient 1 and normal concentration of native factor B plus trace amounts of Bb in patient 2. The depletion of C3 in both patients was due to uncontrolled activation caused by complete factor I deficiency (patient 1) and circulating C3 nephritic factor (patient 2). Both parents of patient 1 had factor I concentrations below (mean-2 SD) that seen in normal healthy individuals (n = 20). Circulating immune complexes (IC) were demonstrated in patient 1 only, whereas serum from both patients had strongly reduced capacity to solubilize preformed IC.