CD34 is a cell surface sialomucin that has been the subject of extensive interest, largely based on its use as a marker for hematopoietic stem cells (HSCs) and vascular endothelia. Despite the almost ubiquitous use of CD34 as a HSC marker, little is known about its cellular function. Our lab was the first to show that CD34 is also highly expressed on mature murine mast cells, and we and other groups have found it to be expressed on eosinophils and dendritic cells. We found that mast cells derived from Cd34-/- mice exhibit a marked increase in cell-cell aggregation. Moreover, when Cd34-/- mice were challenged in a mouse model of asthma, immune cell accumulation in the lung was drastically reduced, while the number of immune cells in the lung at baseline was similar to that of their wild-type counterparts. We have since found that deletion of the Cd34 gene in mice renders these animals resistant to a wide range of other mucosal inflammatory diseases, including hypersensitivity pneumonitis (HP), ulcerative colitis, salmonella infection and intestinal tumor development. Our objectives are to examine the specific role of CD34 in cellular function and to see whether or not CD34 is a viable therapeutic target to treat mucosal inflammatory diseases.