CCL21 and chronic obstructive pulmonary disease: Firstly, we identified eight immune-infiltrating cells closely associated with COPD (Macrophages M0, Macrophages M1, Monocytes, Neutrophils, NK cells activated, Plasma cells, T cells CD4 memory resting, and T cells CD8) and found that these cell types were correlated with eight chemokines (CCL19, CCL20, CCL21, CCL24, CCL27, CCL7, CXCL12, CXCL3).