A subclass of lectins, called selectins are a family of cell adhesion molecules. Selectins mediate the binding of leukocytes to the walls of the blood vessels. This is an essential first step in white blood cell migration into tissue. The binding of selectin to the white blood cell generates signals that can result in initiating the inflammatory response. Once activated the cascade eventually results in a full blow inflammatory response.
Inflammation is a physiological response necessary for survival. In the absence of selectin inflammatory cells can not adhere to activated cells lining the blood vessels when blood flow is sufficiently high. Thus, selectins initiate inflammation on wall of blood vessels. Under-production of selectins is equivalent to an immune deficiency while over-production can mimic many inflammatory diseases.
During dissemination of cancer cells, adhesion to the blood vessel wall, which is essential for metastatic cells to migrate to distant sites, may involve selectins. The expression of these molecules has been reportedly increased on cells of the small vessels adjacent to primary colon and rectal tumors. Since selectins can bind epidermal growth factor, which in turn has been shown to bind with the blood type A antigen, this may be a possible explanation why individuals who are type A have been shown to have higher incidences of several common cancers.
Cell adhesion mediated by selectins and their carbohydrate ligands is involved in the adhesion of cancer cells to endothelial cells during the course of hematogenous metastasis of cancer. In patients with leukemia, this adhesion is involved in the extravascular infiltration of leukemic cells. Extravasation and tissue infiltration of malignant cells in patients with adult T-cell leukemia is mediated by the interaction of selectins and their carbohydrate ligand sialyl Lewis X, which is strongly and constitutively expressed on the leukemic cells. Constitutive expression of Lewis X in these cells is due to the transcriptional activation of Fuc-T VII, the rate-limiting enzyme in the sialyl Lewis X synthesis, induced by the Tax protein encoded by the human T-cell leukemia virus-1, the etiological virus for this leukemia. This is a good example corroborating the proposition that the abnormal expression of carbohydrate determinant at the surface of malignant cells is intimately associated with the genetic mechanism of malignant transformation of cells.()
One of the selectins, the IgE binding protein Epsilon BP, can mediate the binding of IgE to a diverse number of different tissues, including the insulin producing Langerhans cells of the pancreas. This lectin appears to bind cells that carry the A or B blood type antigen over cells that carry the H (O blood type) antigen (). Since this lectin activates masts cells that can then degranulate, releasing cytotoxic inflammatory chemicals, it may help explain why juvenile diabetes appears to have a preference for A and B blood types over type O.