CD4 and infection: Some studies have found that lung-derived CCR9 + CD4 + T cells would be increased after infection, and the small intestinal epithelium could express CCL25, which could promote CCR9 + CD4 + T cells to enter the small intestine, and CD4 + T cells to enter the small intestine would lead to intestinal immune injury, causing a series of mucosal reactions and chronic enteritis (Stenstad et al., 2006; Wang et al., 2014).