FH autoantibody blocks C-terminal recognition function of FH. (A) Patient-derived IgG, added in the indicated concentrations, reduced binding of human C3b to immobilized FH in an ELISA assay, when compared with control IgG. Data represent mean plus or minus SD from 3 experiments. Difference between samples was analyzed by Student t test. *P < .05. (B) Plasma of patient no. 564 (●) caused dose-dependent lysis of sheep erythrocytes, whereas normal human plasma (○) showed no effect. Hemoglobin release was measured as described in “Patients, materials, and methods.” Mean plus or minus SD of data from 5 measurements is shown. (C) Addition of excess FH rescued sheep erythrocytes from complement-mediated lysis. Cells were incubated in 40% no. 564 plasma without FH added and in the presence of the indicated amounts of purified FH, and hemoglobin release was measured. Erythrocyte lysis in the absence of FH was set to 100%. Mean plus or minus SD of data from 4 independent experiments is shown.