receptor

A receptor is like a lock built into a cell, and a drug or natural signal molecule is the key. Only a key with the right shape and chemistry fits the lock, and turning it opens a door — the cell responds. The lock itself does nothing until the right key arrives, which is why a receptor is the place where many drugs begin their action.

More precisely, a receptor is a protein (or sometimes a protein complex) that recognizes and binds a specific chemical signal, then converts that binding event into a change inside the cell. Binding is governed by affinity — how tightly the molecule sticks — while the size of the cellular response depends on the bound molecule's ability to activate the receptor. Receptors evolved to detect the body's own hormones and neurotransmitters; drugs simply borrow these same recognition sites.

Receptors come in major structural families: ligand-gated ion channels, G-protein-coupled receptors, kinase-linked receptors, and nuclear receptors. Each transduces its signal on a different timescale, from milliseconds for ion channels to hours for nuclear receptors that change gene expression.

A useful caveat: not every drug target is a classical receptor. Enzymes, transporters and ion channels are also drug targets, and pharmacologists often reserve the word 'receptor' for proteins whose normal job is to receive a chemical signal, rather than for any protein a drug happens to bind.