APP and Alzheimer disease: Taken together, Aβ42 mainly existed as a monomer in the blood at the early stage of AD (e.g., 3-month-old APP/PS1 mice with less Aβ plaque load and Iba 1-positive cells staining, Figures 3F–H), whereas it appears as oligomers in blood and brain extracts at the late state (e.g., 9-month-old APP/PS1 mice with more Aβ plaque load and Iba 1-positive cells staining, Figures 3F–H).