Bapineuzumab is a humanized antibody developed by Pfizer and Johnson & Johnson 
targeting the amyloid (Aβ) plaques that underlie Alzheimer's disease 
neuropathology. Here we report the crystal structure of a Fab-Aβ peptide complex 
that reveals Bapineuzumab surprisingly captures Aβ in a monomeric helical 
conformation at the N-terminus. Microscale thermophoresis suggests that the Fab 
binds soluble Aβ(1-40) with a K(D) of 89 (±9) nM. The structure explains the 
antibody's exquisite selectivity for particular Aβ species and why it cannot 
recognize N-terminally modified or truncated Aβ peptides.