Complementary peptide having at least a sequence complementary to a major immunogenic region of an acetylcholine receptor involved in myasthenia gravis, characterized in that the complementary peptide has at least a sequence SEQ. ID. NO1 with a tryptophan in position 8, carrying at least one optionally substituted hydrocarbon group. Therapeutic composition comprising the Complementary peptide according to the invention and use thereof for manufacturing a vaccine to be used in a therapeutic or prophylactic treatment of myasthenia gravis in mammals.