CD4 and hypersensitivity pneumonitis: Several changes in T cell subsets are found in fibrotic HP, which may contribute to the non-resolution of inflammation triggering a fibrotic response, including a decrease of the immunoregulatory and antifibrotic γδ T cells, an increase of CD4+ cells, and a switch from a predominant TH1-like phenotype to a TH2-like phenotype (4, 19).