An object of the present invention is to provide a safe and effective method for enhancing an immune response and a medicament for preventing or treating Alzheimer disease comprising amyloid ² peptide that induces an enhanced immune response. An amyloid ² peptide or a portion thereof with addition or insertion of cysteine and a method for enhancing an immune response using the peptide or a method for enhancing an immune response using the peptide together with an adjuvant. A medicament for preventing or treating Alzheimer disease comprising an amyloid ² peptide or a portion thereof that induces an enhanced immune response. A DNA vaccine, that may have the same effect, comprising the gene encoding an amyloid ² peptide or a portion thereof that induces an enhanced immune response with addition or insertion of cysteine.
