C3 nephritic factor (C3NeF): dissociation of cell-bound and fluid phase stabilization of alternative pathway C3 convertase.

Fluid phase C3 conversion by C3 nephritic factor (C3NeF) is usually easily detectable and forms the basis of the C3NeF screening test. We have described a group of patients with membranoproliferative glomerulonephritis or partial lipodystrophy and hypocomplementaemia who have an unusual C3NeF which stabilizes cell-bound C3 convertase of the alternative pathway (C3bBb) but causes such weak fluid phase C3 conversion that a C3NeF screening test is negative. These patients have low concentrations of C5 in serum.