C3 inactivating factor in the serum of a patient with chronic hypocomplementaemic proliferative glomerulo-nephritis.

The serum of a patient with proliferative glomerulo-nephritis and persistent hypocomplementaemia, with low C3 and high C4 levels, was found to cause electrophoretic conversion of C3 when mixed and incubated with normal serum. This activity was found to resist heat at 56° for 30 minutes, and to be due to a fraction of the patient's IgG with a high content of IgG. The effect of this fraction on normal human serum was to cause a decrease in the total haemolytic titre with considerable electrophoretic conversion of C3, while barely affecting functions ascribed to the action of the early complement components.