A ligand, or signaling molecule, is a substance released by a signaling cell that binds to a receptor protein, initiating its activation. The term “ligand” typically refers to a smaller molecule that interacts with a larger one. Signaling molecules are generally smaller than the receptors they bind to and exhibit a high degree of specificity in their interactions. Ligands may include various types of substances, such as:
- Hydrophilic biomolecules, including certain proteins,
- Hydrophobic biomolecules, such as lipids,
- Gaseous molecules, like nitric oxide (NO).
