A composition includes aluminum hydroxide gel and a conjugate of at least one CysA13(33-40) peptide linked to the keyhole limpet hemocyanin (KLH). Maleimidobutyric acid Nhydroxysuccinimide ester (SM) serves as cross-linking agent. The composition can produce an effective and specific immune response against Aβ40. The antibodies produced are specific for Aβ40 without significantly binding to Aβ42. The composition can increase the response against Aβ40 compared with the response produced by other conjugates that include CysAβ(33-40) peptide and KLH, and are bound or conjugated by other crosslinking agents.