The invention relates to an isolated antibody, or fragment thereof, having high affinity for human Aβ protofibrils. The invention further relates to compositions that include the antibody, or a fragment thereof, and a pharmaceutically acceptable buffer. The invention further relates to a method of preventing or treating Alzheimer's disease, which includes the step of administering to a patient having or suspected of having Alzheimer's disease such an antibody, or fragment thereof or a composition that includes the antibody or a fragment thereof.
