C4 nephritic factor in a patient with chronic glomerulonephritis.

C4 nephritic factor (C4NeF) was found in the serum of a patient with chronic glomerulonephritis, whose C3 level was quite low (less than 1% of normal). C4NeF prevented the intrinsic decay of C4b2a enzyme and prolonged the half life of the C4b2a from several minutes to more than 1 h in the solid and fluid phase. The C4NeF activity was associated with a protein fraction immunochemically identified as the IgG3 subclass and it bound specifically to the EAC142 cells. In addition, the patient's serum contained C3-cleaving activity, probably C4b2a, C4NeF complex, which may have lowered the level of C3 in the patient's serum. Spontaneously, the titer of C4NeF decreased, while the level of C3 increased.