Sialyl-LewisX

Sialyl-Lewis X is a carbohydrate ligand (a sialylated and fucosylated tetrasaccharide) displayed on glycoproteins and glycolipids of the leukocyte cell surface. During inflammation, cytokines such as IL-1, TNF-α, and histamine induce endothelial cells to express P-selectin (from Weibel-Palade bodies) and E-selectin (newly synthesized), while leukocytes constitutively express L-selectin. Sialyl-Lewis X on leukocytes interacts with these selectins to produce transient, low-affinity binding, causing the leukocyte to 'roll' along the endothelial surface—this is the first step of the leukocyte adhesion cascade, preceding firm adhesion (via integrins like LFA-1 binding ICAM-1) and transmigration (diapedesis) through the endothelium into tissue. This process is essential for directing leukocytes to sites of infection or injury. Clinically, deficiency in fucosylation of sialyl-Lewis X (due to a defect in fucosyltransferase) causes Leukocyte Adhesion Deficiency type II (LAD II), a rare immunodeficiency characterized by recurrent bacterial infections, marked leukocytosis, and the Bombay blood phenotype, along with developmental delay and short stature. This is distinct from LAD I, which involves a defect in the integrin CD18 subunit affecting firm adhesion. Understanding sialyl-Lewis X is high-yield for USMLE Step 1 in the context of the leukocyte extravasation cascade (rolling, adhesion, transmigration, chemotaxis) and its associated primary immunodeficiencies.