What is a ligand? what do ligands have to do with receptor-mediated endocytosis?

In biochemistry, aligand is a substance that forms a complex with a biomolecule to serve abiological purpose. In protein-ligandbinding, the ligand is usually a molecule which produces a signal by binding to a site on a target protein.

Receptor mediated endocytosis is an endocytotic mechanism in which specific molecules are ingested into the cell. The specificity results from a receptor-ligand interaction. Receptors on the plasma membrane of the target tissue will specifically bind to ligands on the outside of the cell. An endocytotic process occurs and the ligand is ingested.

A very well studied example of receptor mediated endocytosis involves the metabolism of cholesterol.