Abstract: An antigenic peptide comprises the structure X1-X2-X3-X4-X5-X6-X7-P-X9-X10-X11-X12 and is derived from amino acids 113-124 of alpha synuclein. Ac-cording to the structure: P is proline; X1 is L, K, A or S, wherein L is leucine, K is lysine, A is alanine and S is serine; X2 is E or S, wherein E and S are as defined above; X3 is D, E, K, N, A or S, wherein N is asparagine, D is aspartic acid and D, E, K, A and S are as defined above; X4 is M, A, S, L or K, wherein M is methionine and A, S, L and K are as defined above; X5 is P or A as defined above; X6 is V, A or S, wherein V is valine and A and S are as defined above; X7 is D or S as defined above; X9 is D or A as defined above; X10 is N, S or A, wherein N, S and A are as defined above; X11 is E, A or S, wherein E, A and S are as defined above; X12 is present or not and, if present, is A, K, V, S, or G wherein G is glycine and A, K, V and S are as defined above.