Cell adhesion
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").
Since cells are not often found in isolation, rather they tend to stick to other cells or non-cellular components of their environment, a fundamental question is: what makes cells sticky? Cell adhesion generally involves protein molecules at the surface of cells, so the study of cell adhesion involves cell adhesion proteins and the molecules that they bind to.

# Cytoskeletal interactions
For a cell adhesion protein like the one shown in the diagram, the intracellular domain binds to protein components of the cell's cytoskeleton. This allows for very tight adhesion. Without attachment to the cytoskeleton, a cell adhesion protein that is tightly bound to a ligand would be in danger of being hydrolyzed by extracellular hydrolytic enzymes.  This will rip out the adhesion protein from the fragile cell membrane. Often the connection between the cell adhesion proteins and the cytoskeleton is not as direct as shown in the diagram. For example, cadherin cell adhesion proteins are typically coupled to the cytoskeleton by way of special linking proteins called "catenins".

# Adhesion in Prokaryotes
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).

# Adhesion in Viruses
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.

# Adhesion in Eukaryotes
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.

## Human Genetic Diseases
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 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.