Molecules that activate (or, in some cases, inhibit) receptors can be classified as hormones, neurotransmitters, cytokines, and growth factors, in general called receptor ligands.
As moléculas que activan (ou, en certos casos inhiben) os receptores poden clasificarse como hormonas, neurotransmisores, citocinas, e factores de crecemento, e todos eles se denominan ligandos de receptores.