Thus, we may hypothesize that a more widespread use of BALF analysis and uniformed lymphocyte phenotyping might help to dissect the ongoing lung inflammatory processes (e.g., presence of a CD4+ alveolitis, B cell increase and activation, mediators potentially acting as activity biomarkers) and to potentially define tailored treatments that, at present, are provided only by histologic evaluation. The gene discussed is CD4; the disease is hypersensitivity pneumonitis.