CD4 lymphocyte predominance and increased CD4/CD8 ratios on the BAL have been documented in chronic HP, when compared with acute and subacute forms but the use of these markers to diagnose or exclude the disease in the clinical setting is limited by inconsistent findings and significant variation with external factors [9,62,63,64,65,66,67,68,69,70]. This evidence concerns the gene CD8A and hypersensitivity pneumonitis.