Mechanisms of disease

3. Targets for action of medicines

3.4. Missing ligands

Finally, in some diseases the target is undisturbed and intact, but the internal ligand that binds to the target is missing. Therefore, homeostasis cannot be achieved. The ligand could be a small molecule like a metabolite, a hormone, a transmitter (e.g. dopamine), a vitamin, a nutrient (e.g. amino acids) or an ion (e.g. iron).  It could also be a large molecule like a hormone (e.g. insulin, thyroxin, corticosteroids). If it is possible to produce the ligand, ‘substitution therapy’ can be used. This therapy replaces the natural ligand in the body. For example, use of synthesised insulin in the treatment of diabetes. This can also apply to vaccines and cells (e.g. red blood cells (by transfusion) or stem cells (by transplantation)).