Receptor

In cell biology and pharmacology, a receptor is a protein that recognizes and binds a particular signaling molecule, called its ligand, and responds by changing what the cell does. Ligands include hormones, neurotransmitters, growth factors and many drugs. The fit between ligand and binding site is often compared to a key in a lock, so each receptor responds to one signal or a small family of related ones.

Most receptors sit in the plasma membrane and relay a signal from outside to inside. Ion channel-linked receptors open a pore for ions, G protein-coupled receptors activate a G protein that passes the message on, and enzyme-linked receptors, such as the insulin receptor, act as or switch on enzymes. Small fat-soluble signals such as steroid and thyroid hormones instead cross the membrane and bind receptors inside the cell, which then act directly on genes.

Many drugs work through receptors, either activating them as agonists or blocking them as antagonists, and long use can raise or lower the number of receptors a cell makes. In physiology the word also names sensory receptors, the cells or nerve endings that detect stimuli such as light, touch or heat.