Ligand is a substance that forms a complex with a biomolecule to serve a biological purpose.
It acts as a signal triggering molecule, binding to a site on a target protein.
The binding is usually reversible.
Ligands may be substrates, inhibitors, activators, and neurotransmitters.
A ligand that binds to a receptor triggers a physiological response. The site where the ligand is bound is known as the binding or active site of the protein.
The binding of a ligand by a protein is generally quite specific.