CD4 and hypersensitivity pneumonitis: In a study of 63 patients with HP, 13 with IPF, and 113 controls, a significant positive correlation was found between periostin levels and serum KL-6 levels, CD4/CD8 ratio in bronchoalveolar lavage fluid, and fibrosis score on HRCT, and a significant negative correlation was found between periostin levels and carbon monoxide diffusing capacity.