The present invention relates to prevention and treatment of Alzheimer's disease (AD). More specifically, the invention relates to use of a non-wild type protofibril or compound(s) with protofibril forming activity for active immunisation in the purpose of treating or preventing AD. The invention further relates to a peptide, A.beta.-Arc, with high protofibril forming activity as well as several applications thereof, such as antibodies against said peptide for passive immunisation against AD.