Receptor

A receptor is a molecular protein or complex capable of binding a specific molecule, or ligand, and translating that binding into a cellular response. Receptors are distributed on cell surfaces or inside cells and may act as gatekeepers for ions, enzymes, or transcriptional programs. Binding is generally selective and occurs with a characteristic affinity, permitting discrimination among signals even at low concentrations.

Most receptors are transmembrane proteins that perceive extracellular signals such as hormones, neurotransmitters, growth factors, or sensory stimuli. Primary classes include G protein-coupled receptors, receptor tyrosine kinases, and ligand-gated ion channels, as well as intracellular or nuclear receptors that modulate gene expression in response to lipid-soluble ligands such as steroids. Immune receptors, including B cell and T cell antigen receptors, detect specific antigens and initiate adaptive responses. Sensory receptors in the eye, ear, and other organs translate physical energy into neural signals.

Upon ligand binding, receptors undergo conformational changes that trigger intracellular signaling cascades, alter enzyme activity, or modify gene transcription. Pharmacology often treats receptors as targets; ligands may be agonists that activate the receptor, antagonists that block it, or allosteric modulators that modify its response. Receptors can display desensitization, upregulation, or downregulation in response to persistent stimulation, adjusting sensitivity over time.

Receptors are central to physiology, development, and homeostasis, and dysfunction can lead to disease. Understanding receptor structure and signaling underpins drug design and therapeutic interventions.