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Ligand

(Science: chemistry, immunology) Any molecule that binds to another, in normal usage a soluble molecule such as a hormone or neurotransmitter, that binds to a receptor. The decision as to which is the ligand and which the receptor is often a little arbitrary when the broader sense of receptor is used (where there is no implication of transduction of signal). In these cases it is probably a good rule to consider the ligand to be the smaller of the two thus in a lectin sugar interaction, the sugar would be the ligand (even though it is attached to a much larger molecule, recognition is of the saccharide).