Incubation at pH 2.5 induced artifactual ELISA signal only interacts with full length Aβ; Authentic antibodies bind both full length Aβ and N terminal fragments. Epitope mapping of normal sera from unvaccinated mice (panel A;1:400 dilution), mice vaccinated with Aβ1–42 peptide (panel B; 1:400 dilution) or mouse anti-human Aβ1–16 monoclonal antibody (6E10; 4C;50 ng/ml) was performed. Full length human Aβ and human Aβ fragments (singly and in combination as indicated on graph) were used to coat the plate and identify the domain that interacted with the IgG fractions following pH 7 preincubation (full length Aβ only) or incubation at pH 2.5 for 20 minutes at RT. After centrifugation and neutralization the samples were then incubated on ELISA plates coated as indicated. Values are mean ± SEM. ** P < 0.001 compared to pH 2.5 plate coated with Aβ1–42 peptide.