The variable incidence of HP, typically occurring within the first year of use, may suggest that the disease is an idiosyncratic immune reaction rather than a dose-dependent toxic injury.8 It is a hypersensitivity alveolitis characterised by T cell-mediated (CD4 and CD8) proliferation and the proliferation of type 2 alveolar cells, resulting in cytokine release.23 Although risk factors contributing to the development of the disease have been identified, the exact extent to which these factors contribute to the clinical manifestation remains unclear. This evidence concerns the gene CD4 and hypersensitivity pneumonitis.