CD28 and immune system disorder: Among patients with AECOPD, the total number of lymphocytes, the percentage of lymphocytes, and the CD4+/CD8+ T-cell ratio in the peripheral blood were significantly lower, while the proportions of negative regulatory cells (CD4+CD27-CD28- T cells, CD4+CD39+ T cells, and CD4+CD25+FoxP3+ T cells) were significantly higher, suggesting cellular immune suppression and immune dysfunction in AECOPD patients.