Cellular adhesion

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Cellular adhesion is the binding of a cell to another cell or to a surface or matrix. Cellular adhesion is regulated by specific adhesion molecules that interact with molecules on the opposing cell or surface. Such adhesion molecules are also termed receptors and the molecules they recognize are termed ligands (and sometimes counterreceptors).

Prokaryotes have adhesion molecules usually termed adhesins. Adhesins may occur on pili (fimbriae), flagellae, or the cell surface. Adhesion of bacteria is the first step in colonization and regulates tropism (tissue- or cell-specific interactions).

Viruses also have adhesion molecules required for viral binding to host cells. For example, influenza virus has a hemagglutinin on its surface that is required for recognition of the sugar sialic acid on host cell surface molecules. HIV has an adhesion molecule termed gp120 that binds to its ligand CD4, which is expressed on lymphocytes.

Eukaryotic protozoans also express multiple adhesion molecules. An example of a pathogenic protozoan is the malarial parasite (Plasmodium falciparum), which uses one adhesion molecule called the circumsporozoite protein, to bind to liver cells, and another adhesion molecule called the merozoite surface protein to bind red blood cells. In human cells, which have many different types of adhesion molecules, the major classes are named integrins, Ig superfamily members, cadherins, and selectins. Each of these adhesion molecules has a different function and recognizes different ligands. Defects in cell adhesion are usually attributable to defects in expression of adhesion molecules.

There are human genetic diseases caused by inability to express a specific adhesion molecule. An example is leukocyte adhesion deficiency-I (LAD-I), where patients do not express the beta-2 integrin subunit precursor. This integrin is required for leukocytes to adhere to the blood vessel wall during inflammation in order to fight infection. The leukocytes from LAD-I patients fail to adhere and patients exhibit serious episodes of infection that can be life threatening.