Sialyl Lewis X (SLeX)

is a tetrasaccharide carbohydrate that is usually attached to O-glycans on the surface of cells. It is known to play a vital role in cell-to-cell recognition processes. Sialyl Lewis ^X is one of the most important blood group antigens and is displayed on the terminus of glycolipids that are present on the cell surface. The Sialyl Lewis ^X determinant, E-selectin ligand carbohydrate structure, is constitutively expressed on granulocytes and monocytes and mediates inflammatory extravasation of these cells.

Resting T and B lymphocytes lack its expression and are induced to strongly express sialyl Lewis ^X upon activation. The Sialyl Lewis ^X determinant is expressed preferentially on activated Th1 cells but not on Th2 cells. Defective synthesis of the sialyl Lewis ^X antigen results in immunodeficiency. Defective synthesis can be caused by the loss of fucosyltransferase, impairing the glycosylation of the glycosphingolipid.