Autoantibody reactivity with short factor H fragments. Autoantibody binding to FH fragments (corresponding to SCRs 1–7, 8–15, 16–18, and 19–20) and a FH–related protein 1 fragment (SCR 4–5). In 31% of patients the antibodies were polyclonal. In 69% of patients the antibodies were monoclonal, and in 91% of these the binding was to SCRs 19–20. Two of the 3 patients with 2 copies of CFHR1 had antibodies that bound predominantly to SCRs 1–7. FHR1, factor H–related protein 1; ID, identification; NCL, Newcastle; RU, relative units; SCR, short consensus repeat.