Correlation Between Vitamin D, Inflammatory Markers, and T Lymphocytes With the Severity of Chronic Obstructive Pulmonary Disease and its Effect on the Risk of Acute Exacerbation: A Single Cross-sectional Study.

Purpose: Chronic obstructive pulmonary disease (COPD) will become the fourth largest cause of death of chronic diseases in the world in 2030. The incidence of COPD ranked top among chronic diseases in the world. At present, there is a lack of simple and effective drugs for the treatment of COPD and for slowing the progression of the disease.