Although CD34 facilitates the migration of several hematopoietic effector lineages (mast cells, eosinophils and DCs) and Cd34−/− mice are protected in models of asthma, ulcerative colitis and hypersensitivity pneumonitis, this likely reflects a delay, but not a block, in the ability of CD34+ effector cells to migrate[16,18,28,29]. This evidence concerns the gene CD34 and hypersensitivity pneumonitis.