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Receptors are proteins or protein complexes that bind specific signaling molecules, known as ligands, to initiate cellular responses. They are found in both plasma membranes and intracellular compartments and play central roles in cell communication, homeostasis, immunity, and development. Receptor types are broadly classified by the nature of their binding site, signaling mechanism, and subcellular localization.

Ionotropic (channel) receptors form or regulate ion channels that open in response to ligand binding. Binding typically causes rapid depolarization or hyperpolarization of the membrane potential. Classic examples include nicotinic acetylcholine receptors and AMPA glutamate receptors. Because the response is nearly instantaneous, ionotropic receptors are especially important in fast synaptic transmission in the nervous system.

Metabotropic receptors are G‑protein‑coupled receptors (GPCRs) and other receptors that engage secondary messenger systems rather than directly controlling ion flow. Upon ligand binding, they activate heterotrimeric G proteins, leading to the production of cyclic AMP, inositol trisphosphate, or other second messengers. GPCRs form the largest receptor family in humans and are involved in vision, smell, hormone signaling, and many pharmacological targets.

Enzyme‑linked receptors possess intrinsic or associated enzymatic activity. The epidermal growth factor receptor (EGFR) is a tyrosine kinase that autophosphorylates and initiates cascades involving MAP kinase and PI3K/AKT. This class includes receptor tyrosine kinases, receptor serine/threonine kinases, and others that transduce extracellular signals to the cytoplasm and nucleus.

Nuclear hormone receptors reside predominantly in the cytoplasm or nucleus and bind lipophilic ligands such as steroids, thyroid hormones, or retinoic acid. Ligand binding alters their conformation and DNA‑binding affinity, directly regulating transcription of target genes.

In many cases, receptor families overlap or interact. For instance, membrane GPCRs can modulate ion channel activity through signaling pathways, while ion channel activity can influence intracellular calcium levels that feedback to nucleus‑bound receptors. The study of receptor biology is essential for developmental biology, pharmacology, and the design of targeted therapeutics.