HLA-DQA1 and chronic obstructive pulmonary disease: Higher HLA-DQA1 and HLA-DR expression levels have been associated with frequent exacerbators in COPD patients, while serum-soluble HLA class II molecules have shown negative correlations with lung function indices such as forced expiratory volume in 1 s (FEV1) value and FEV1/forced vital capacity (FVC) ratios.