(A) Resting endothelium is protected against complement activation by complement regulators. (B) Normal serum deposited small amounts of C3b (and C3b degradation products) and C5b-9 on ADP-activated endothelial cells, but complement regulatory proteins presented in normal serum inhibited propagation of complement activation on the surface of endothelium. (C) In the presence of serum from a patient with acute aHUS, complement regulation on endothelial cells is ineffective, resulting in a larger number of C5b-9 complexes deposited on the surface of resting endothelium. (D) Serum samples from patients with acute aHUS, aHUS in remission, and healthy carriers of complement mutations deposited C5b-9 on the surface of ADP-activated endothelial cells. In this assay, HMEC-1 were used as the source of endothelial cells.