SNCA and Parkinson disease: Importantly, T cells with specificity for epitopes of alpha-synuclein have been identified and reported to occur at higher frequency in PD than controls; furthermore, their frequency was closely associated with possession of known PD risk alleles at the HLA locus,29 thus suggesting that alpha-synuclein may drive a peripheral adaptive immune response as well as an innate response of microglia in the brain.