HLA-G and chronic obstructive pulmonary disease: According to Kendall’s tau-b correlation coefficient test, the severity of COPD is positively correlated with β2-microglobulin (Kendall’s tau-b=0.790, P<0.05), and negatively correlated with calcium, chloride, potassium, and sodium levels (Kendall’s taub=-0.751, -0.774, -0.757, -0.768, P<0.05).