A ligand is a molecule that specifically binds to another molecule, often a larger one. Ligand binding usually causes a receptor protein to undergo a change of shape, which often activates the receptor. After activation, the receptor can then interact with other cellular molecules.  Ligands do not only bind to proteins, however, and can be larger than the molecules that they bind to.
- Campbell, Neil A., and Jane B. Reece. Biology. San Francisco: Benjamin Cummings, 2002. Print.