The present invention relates to immunogenic but non-depositing-forming polypeptides or peptides homologous to amyloid .beta., prion, amylin or .alpha.-synuclein which can be used alone or conjugated to an immunostimulatory molecule in an immunizing composition for inducing an immune response to amyloid .beta. peptides and amyloid deposits, to prion protein and prion deposits, to amylin and amylin deposits, to .alpha.-synuclein and deposits containing .alpha.-synuclein, or to polyglutamine repeats and deposits of proteins containing polyglutamine repeats. Described are also antibodies directed against such peptides, their generation, and their use in methods of passive immunization to such peptides and deposits.