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Receptors are proteins or protein complexes that detect signals and translate them into cellular responses. They form the core of cellular communication, binding specific ligands such as neurotransmitters, hormones, or environmental cues. Receptors are broadly categorized as cell-surface receptors and intracellular receptors.

Cell-surface receptors reside in the plasma membrane and include ligand-gated ion channels, G protein-coupled receptors (GPCRs), and receptor tyrosine kinases (RTKs). Ligand-gated ion channels open or close in response to ligand binding, altering ion flow. GPCRs activate intracellular G proteins, triggering second messenger systems. RTKs phosphorylate target proteins upon ligand binding, initiating signaling cascades that control growth, metabolism, and survival. Intracellular receptors, located in the cytoplasm or nucleus, primarily bind lipophilic ligands (such as steroid hormones) that modulate gene transcription.

Ligand binding is governed by affinity and specificity, often described by dose–response relationships. Receptors can be regulated by desensitization, internalization, downregulation, or synthesis, affecting sensitivity. Receptor signaling is integrated through cross-talk between pathways.

Clinically, receptors are major drug targets. Agonists activate receptors; antagonists block them; allosteric modulators modify receptor responses. Receptor dysfunction is implicated in numerous diseases, including asthma, diabetes, cardiovascular disorders, and neurodegenerative conditions.

Studying receptors involves binding assays, structural biology, and genetic models to understand their roles and to develop therapeutics. Receptors thus serve as essential mediators of communication between cells and their environment.