Receptors are typically glycoproteins located on or within cell membranes that specifically recognise and bind to ligands. Ligands are signalling molecules which serve as chemical messengers within or between cells. Ligand binding results in conformational change of the receptor leading to a series of biochemical reactions inside the cell to elicit a response. Formation of a ligand-receptor complex can be reversible or irreversible. The main different types of receptor include ligand-gated ion-channel receptors, G-protein coupled receptors, kinase-linked receptors and intracellular receptors, amongst others.