The study group enrolled patients with OSA, fulfilling the following requirements

1. 20~60 years ago, female or males
2. Agree participate the study and sign informed consent
3. Has not received any treatment for OSA
4. No obvious comorbidities (including autoimmune diseases)
5. No history of sarcoidosis and tuberculosis
6. No use of steroid and immunosuppressant

The control group enrolled age-, gender- and BMI-matched individuals without OSA, fulfilling the following requirements

1. 20~60 years ago, female or males
2. Agree participate the study and sign informed consent
3. No obvious comorbidities (including autoimmune diseases)
4. No history of sarcoidosis and tuberculosis
5. No use of steroid and immunosuppressant

This study aims to compare the difference in the number and function of regulatory T cells between OSA patients and controls. The investigators will draw peripheral venous blood samples from both groups and measure the number and function of their regulatory T cells. However, some, NOT ALL, of OSA patients may receive continuous positive airway pressure (CPAP) treatment-the standard therapy for severe-to-moderate OSA. Use of CPAP is based on physician's suggestion and patients' decision, but not for this study. The devices are provided by patients themselves and its type/brand is not limited or prespecified. For those with CPAP therapy, their blood sample will be re-collected 26 weeks later to repeat measurement for subgroup analysis.