Parkinson's disease (PD) is an age-related neurodegenerative movement disorder of unknown aetiology. Immune abnormalities have been described in PD including the occurrence of autoantibodies against neuronal structures and high numbers of microglia cells expressing the histocompatibility glycoprotein human leucocyte antigen-DR in the substantia nigra. An infectious cause for PD has been discussed for years. Disturbed cellular and humoral immune functions in peripheral blood of patients with PD have been also reported. An elevated gd+T cell population and increased immunoglobulin G immunity in CSF to heat shock proteins have been found in PD. Cytokines and apoptosis-related proteins were elevated in the striatum in patients with PD. Activated glial cells may participate in neuronal cell death in PD by providing toxic substances. We may conclude that the immune system is involved in the pathogenesis of PD. However, we are not able to determine whether the disturbances described above constitute a primary or secondary phenomenon. Immunomodulatory agents may have important applications in the development of new therapies for PD.