Like a key to a lock, a ligand is a molecule that fits perfectly inside its specific receptor type. Both ligands and receptors exist on the surface of cells in the body and transmit a signal when they connect. This signal starts a biological change specific to the type of ligand and receptor interaction. A Fas ligand is a protein that exists on cell walls and binds to Fas receptors. When the Fas ligand binds to the Fas receptor, the cell containing the Fas receptor dies through a process called programmed cell death.
Programmed cell death, also called apoptosis, is one of the main components of the immune system response in the animal kingdom. The majority of cells in the body express Fas receptors and are thus sensitive to the Fas ligand-triggered apoptosis. Fas ligand, on the other hand, is almost solely expressed on activated T-cells, one of the main types of cells of the immune system.
When a cell dies from damage and not apoptosis, the infected or damaged DNA is able to leave the cell intact and interact with other cells, thereby possibly spreading the infection. Apoptosis shreds DNA, purges the cell’s contents, and sacrifices the cell, lending it unable to infect other cells. The Fas ligand triggers programmed cell death when it binds to the Fas receptor, so it is vital to an organism’s longevity.
Numerous health-ensuring functions of the human body have been linked to healthy Fas ligand and receptor functioning. Some examples of this are in killing infected cells, killing aging cells and maintaining a proper immune system cell balance in the body. In the latter case, the Fas ligand-triggered apoptotic cycle ensures that the body does not create too many immune system cells. If too many immune system cells are created, they might attack the organism’s own healthy cells, creating too much damage. This is sometimes the case in an autoimmune disorder.
Malfunctions in the Fas binding system have been implicated in neurodegenerative disorders, cancer, acquired immunodeficiency syndrome (AIDS) and drug resistance in tumors. Following chemotherapy, some tumors develop an over-expression of Fas ligands on their cell surfaces, which results in the cancer cells killing off the beneficial immune system cells that the body would otherwise use to fight the cancer. Fas ligands are a major area of study in cancer research because of their role in apoptosis and cancer.