IVIG protects primary hippocampal neurons against Aβ1-42 toxicity. Freshly solubilized 5 μM Aβ1-42 (Aβ oligomer rich preparation) was incubated for 0 or 48 h, cross-linked to preserve the peptide structure and analyzed with western blotting with an anti-Aβ antibody (6E10). Freshly solubilized Aβ1-42 consisted of monomers, dimers, various forms of oligomers and aggregates with larger molecular weight (Mw). After 48 h of incubation, low Mw forms of Aβ1-42 were reduced while high Mw aggregates were detected (A). Mw of Aβ1-42 monomer is 4.5 kDa. Mouse hippocampal neurons were exposed to freshly solubilized Aβ1-42. After 48 h of incubation, the cytotoxicity was determined with a defect in cell membrane integrity and the presence of condensed chromatin in nuclei. Aβ1-42 was highly cytotoxic (B, C) in micromolar concentrations (n = 3, p < 0.001). IVIG reduced Aβ1-42-induced neurotoxicity by the terms of cytoplasmic LDH release (B, n = 3, p < 0.05) and the number of apoptotic/necrotic cells (C, n = 3, p < 0.001). The effect of IVIG on Aβ fibrillization was studied by incubating freshly solubilized 10 μM Aβ1-42 in the presence of IVIG or irrelevant human recombinant IgG (ctrl IgG) for 24 h and quantifying fibrillar Aβ fluorometrically with Thioflavin-T staining. IVIG reduced Aβ1-42 fibrillization (D, p < 0.001, n = 3) but this was not concentration-dependent and also occurred in the presence of an irrelevant IgG (D, p < 0.001, n = 3). To study the effect of IVIG on solubilization of natively formed brain Aβ deposits, unfixed cryostat-cut hippocampal brain sections prepared from aged APP/PS1 mice were incubated with 20 μM IVIG for 7 days. IVIG did not solubilize pre-formed Aβ deposits; the release of Aβ into the medium was not increased (E, p > 0.05, n = 6) while Aβ immunoreactive area in the brain sections was not decreased (F, p > 0.05, n = 6), as determined with ELISA and immunostaining, respectively.