selectins

Selectins are a family of cell adhesion molecules that belong to the C-type lectin superfamily and mediate calcium-dependent adhesion between leukocytes and vascular endothelium. They play a central role in the early steps of leukocyte trafficking during the immune response by supporting transient, low-affinity interactions under shear flow.

There are three members: L-selectin (CD62L) expressed on most leukocytes; E-selectin (CD62E) induced on activated endothelial cells in response to inflammatory signals; and P-selectin (CD62P) stored in Weibel-Palade bodies of endothelial cells and in platelet alpha granules, rapidly mobilized to the surface upon activation.

Selectins recognize specific carbohydrate ligands, such as sialyl Lewis X, presented on leukocytes. L-selectin mediates initial tethering in high endothelial venules, while P- and E-selectin mediate capture and rolling on activated endothelium. These interactions are calcium-dependent and of low affinity, allowing leukocytes to roll along the vessel wall; chemokine signaling then activates integrins to promote firm adhesion and extravasation.

Structurally, selectins are type I transmembrane glycoproteins with an N-terminal lectin domain, an epidermal growth factor–like domain, and a mucin-like stalk, followed by a transmembrane region and cytoplasmic tail. Ligand binding is influenced by glycosylation and conformational changes that modulate affinity.

Regulation of expression differs among family members: E- and P-selectin are upregulated by inflammatory cytokines such as IL-1 and TNF-α, whereas L-selectin is constitutively expressed on leukocytes and can be shed upon activation. Functionally, selectins initiate leukocyte trafficking and contribute to inflammation, host defense, and, in some contexts, tumor cell adhesion and metastasis. They remain a focus of research for anti-inflammatory strategies.