CD14 and silicosis: The expression of PD-1 on CD4+ and CD8+ T cells and the expression of PD-L1 and PD-L2 on CD14+ monocytes in peripheral blood of silicosis patients are significantly lower than those of healthy people, and the downregulation of PD-1 may be related to the single nucleotide polymorphism of PDCD1 gene (170, 171).