Ever since its discovery more than 20 years ago, the CD34 antigen has been widely used as a marker to identify stem cells, precursor cells that give rise to all types of specialized cells. However, the exact function of CD34 expression on hematopoietic precursors and mature cells is still not well understood. Dr. Marie-Renée Blanchet and colleagues have uncovered some fascinating details about the role of CD34 in allergy and asthma. The team recently demonstrated that CD34 is expressed on mature mast cells and eosinophils – two types of cell that respond to injury during inflammation of the body’s tissues – and that the CD34 antigen is involved in their recruitment to the lung and peritoneum. They showed that mice without the CD34 antigen are protected against development of airway hyper-responsiveness and airway inflammation, which are two major hallmarks of allergic asthma. Finally, in preliminary experiments, these mice also showed protection in hypersensitivity pneumonitis, another model of lung inflammation. Now, Blanchet is working to better understand the mechanisms behind these recent findings. Many cell types involved in asthma and hypersensitivity pneumonitis express CD34, some in which the role of this protein remains unknown (eg. fibrocytes and dendritic cells). She plans to use models to elucidate the role of CD34 expression in these cells. Ultimately, she hopes her studies will reveal potential targets for treatment of allergy and inflammation.