The goal of this observational study is to learn if immunosuppressive drug treatment influences the clinical course of Parkinson's disease. Recent research suggests that Parkinson's disease may have an inflammatory background. Numerous studies have identified elevated levels of pro-inflammatory markers in people with Parkinson's disease. Therefore, immunosuppressive drugs may potentially affect the course of this disease.

The investigators will recruit people with Parkinson's disease, who receive long-term immunosuppressive treatment for other chronic illnesses, into the study group. The study will also recruit people with Parkinson's disease with no history of immunosuppressive drug treatment into the control group. The main question this study aims to answer is:

> Does immunosuppressive drug treatment slow down the onset and/or progression of Parkinson's disease?

Researchers will compare Parkinson's disease progression rates and serum inflammatory marker levels between the study and control group, to see if immunosuppressive drug treatment influences the course of Parkinson's disease.

The study requires each participant to attend one hospital appointment. This appointment will be approximately one hour long, and will involve:

1. clinical Parkinson's disease motor assessments conducted by the research team -> participants will be evaluated in the OFF state, meaning they will not have taken their Parkinson's disease medication the day of the assessment;
2. completion of Parkinson's disease symptom scales;
3. a one-time serum sample collection;
4. detailed medical history obtainment from the participant by the research team, with a primary focus on the patient's immunosuppressive treatment history and individual clinical course of Parkinson's disease.

Inclusion criteria for the study group:

1. Diagnosis of Parkinson's disease according to the International Parkinson and Movement Disorder Society Clinical Diagnostic Criteria for Parkinson's disease;
2. Immunosuppressive medication use - medication taken regularly, which we have defined as administered at a dose and frequency effective for the indicated disease, for over 12 months, systemic administration route (oral, intramuscular, subcutaneous, intravenous and inhaled); The most commonly used immunosuppressive drugs that we anticipate will be used by the study group participants include: glucocorticosteroids (GC) - budesonide, cortisone, dexamethasone, hydrocortisone, methylprednisolone, prednisolone, prednisone, triamcinolone; calcineurin inhibitors - tacrolimus, cyclosporine; inosine monophosphate dehydrogenase (IMDH) inhibitors - azathioprine, leflunomide, mycophenolate; mammalian target of rapamycin (mTOR) inhibitors - sirolimus, everolimus; biologics - abatacept, adalimumab, anakinra, certolizumab, etanercept, infliximab, ixekizumab, natalizumab, rituximab, secukinumab, tocilizumab, ustekinumab, vedolizumab; antimetabolites - methotrexate; inclusion of participants taking immunosuppressive drugs not listed above will be considered individually;
3. Provision of informed consent to participate in the study.

Inclusion criteria for the control group:

1. Diagnosis of Parkinson's disease according to the International Parkinson and Movement Disorder Society Clinical Diagnostic Criteria for Parkinson's Disease;
2. No history of regular use of immunosuppressive medications;
3. Provision of informed consent to participate in the study.

Exclusion criteria for the study group:

1. Immunosuppressive drug administration at an ad hoc basis (i.e. when symptoms occur) or irregularly (i.e. drug taken at an inappropriate dose and/or with a regularity ineffective for the indicated disease);
2. Use of immunosuppressive drugs for a period of less than 12 months;
3. Ineffective treatment of the disease requiring the use of immunosuppressive drugs (no improvement/progression of disease symptoms);

Additional exclusion criteria for both the study and control groups:

1. Advanced cancer; severe heart failure; severe renal failure; untreated autoimmune disease; active inflammation; diseases that impair drug absorption/metabolism;
2. Dementia;
3. Failure/inability to provide informed consent.

Participants will be recruited among patients of the Department of Neurology and Parkinsonism Clinic of the Faculty of Health Sciences, Medical University of Warsaw, Warsaw, Poland