Patients with APS and microvascular thrombosis identified by kidney biopsy have normal ex vivo complement activation and lack significant in vivo complement deposits. (A) Ex vivo C5b9 formation on the perturbed endothelium did not differ between patients with APS (n = 15) and normal human serum (NHS), whereas serum from patients with primary aHUS and a pathogenic gain-of-function variant in C3 induced unrestrained complement activation (n = 5); original magnification ×400. **P < .01; NS not significant. (B) IgG2, but not the other subclasses, bound to the endothelium when incubated with serum from triple-positive patients (2 of 2); original magnification ×200. (C) Microvascular thrombosis on kidney biopsy (arrowheads). Scant deposits of C3c (7 of 17) and/or C5b9 (2 of 17) along segments of the glomerular capillary wall were uncommon. Original magnification ×400.