Adhesion molecules.

There are four families of adhesion molecules called selectins, integrins, mucin-like vascular addressins and members of the immunoglobulin superfamily and each family contains many members. Different adhesion molecules bind to each other in a specific manner and enable cells to interact with each other. Cell–cell adhesion is controlled both by the expression of particular adhesion molecules and in some cases by the activation status, or actual binding capacity, of the adhesion molecules.

Adhesion molecules.

Different adhesion molecules are expressed on different cell types; some are expressed constantly on the cell surface and others are induced by cell activation, e.g. by cytokines.
By altering cell-adhesion molecule expression or activity on endothelial cells or leukocytes, it is possible to control whether particular leukocytes bind to endothelium at a particular tissue site and, hence, the entry of the leukocytes into the tissue.
Selectins are glycoproteins that are lectins, i.e. sugar-binding molecules, some of which are expressed on leukocytes and some on endothelial cells.
Mucin-like vascular addressins are heavily glycosylated proteins and therefore can bind to the selectins. Some are expressed on leukocytes and some on endothelial cells.
Integrins are heterodimeric proteins consisting of an α-chain and a β- chain and are expressed on leukocytes. There are many α- and β-chains and they can pair to give many combinations of integrins with different expression and binding specificity. Some integrins will bind to target molecules only following activation of the leukocyte by various factors.
Immunoglobulin superfamily: these molecules contain immunoglobulin (Ig)-like domains (110 amino acids flanked by an intra-chain disulphide bond) and are the binding target for the integrins. They are expressed on endothelial cells.