Sialyl LewisX (sLeX), also known as cluster of differentiation 15s (CD15s) or stage-specific embryonic antigen 1 (SSEA-1), is a tetrasaccharide carbohydrate which is usually attached to O-glycans on the surface of cells.

Sialyl-LewisX is also 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-LewisX determinant, E-selectin ligand carbohydrate structure, is constitutively expressed on granulocytes and monocytes and mediates inflammatory extravasation of these cells.

Leukocytes move through the blood stream and then tether themselves to the endothelial wall and roll along the endothelium before potentially exiting into the tissue.

For sialyl-LewisX to bind to E-selectin, it can be part of either an N-linked or O-linked glycan attached to cell surface glycoproteins such as PSGL-1, CD43 or CD44.

[4] Furthermore, adding purified and solubilized ZP3 or ZP4 from the human oocyte dose-dependently inhibits sperm–ZP binding in the hemizona assay.

Defective synthesis of the sialyl-LewisX antigen results in immunodeficiency (leukocyte adhesion deficiency type 2).

Sialyl-LewisX is being researched with CD markers to find new ways to create biosensors for cancer cells.

In June 2019, before the onset of the COVID-19 pandemic, the receptor for sulfated sialyl-LewisX oligosaccharide (particularly with α2,3 linkages) was found to be the preferred binding site, both in humans and in dromedary camels, for the coronavirus causing Middle East respiratory syndrome (MERS), the sixth coronavirus to be described.

Clusters of differentiation are a naming system devised in 1982 to classify cell-surface antigens on leukocytes identified via monoclonal antibodies.