CD40LG and chronic obstructive pulmonary disease: Therefore, the purpose of our systematic review and meta-analysis was to assess serum, sputum, and bronchoalveolar lavage (BAL) levels of IgG, IgG subclasses, IgA, and IgM in COPD, and the association between immunoglobulins and key clinical outcomes.EMBASE and Ovid MEDLINE were searched from inception to April 2024.